Let me introduce you to tomate frito. It's a Spanish thing, and the nearest most people in the UK get to it is a good neapolitan sauce on their pasta.
I learned to make this in the same way I learn to make most things. I watch somebody do it, ask a few questions to find out what could break it if I messed with the recipe and then I go and do my own thing.
At its most basic it's tomatoes and onions cooked for an eternity then puréed and bottled. I make a tastier version of the plain tomato sauce, which is healthier too. You might have come across it in Italy, where it's known as Pomarola. It has a few extra vegetables which thicken the sauce, give a lot of background taste and stop you feeling so guilty for the piles of pasta you're about to consume when you know you're getting your five-a-day in the sauce.
First things first. The tomatoes have to be the reddest — and therefore ripest — plum tomatoes you can find. I sometimes use big fat vine tomatoes when they're cheap in the veg shop on the corner. These San Marzano beauties came fabulously cheap from The People's Supermarket.

This recipe is based on 2 kilos/4 pounds of plum tomatoes. It sounds like a lot, but it'll make enough sauce for pasta for 4 on the day you make it and for a couple of large jars to keep for later. Unless you live in our house, in which case you'll use it all up over the next couple of days before it gets to see a sterilised jar.
ingredients
2 kg plum tomatoes
1 onion
1 carrot
1 courgette
2 or 3 cloves of garlic
bay leaves
basil, rosemary or thyme
pinch salt and sugar to taste
equipment
a very large pan
knife, chopping board etc.
a food mill, pasapurés, mouli, whatever you call it*
large bowl for pureeing into
sterilised jars
how to make it
Get chopping the tomatoes. If you have sensitive skin you might want to wear gloves. 2 kilos is a lot of tomatoes. I don't bother to peel them or seed them, as all that is dealt with later when you put the sauce through the pasapurés (food mill). If you're not using a food mill then you can't skip this step. (To peel: score them lightly, plunge into boiling water for a minute, then the peel should come off easily.)

Now chop a large onion, peel a few cloves of garlic, peel and chop a large carrot and a courgette and grab a few bay leaves and a small sprig of herbs, whatever you fancy — basil, rosemary or thyme.
Take a large pan and heat a splash of olive oil. Add the onions and a pinch of salt and allow to soften. Add all the other vegetables, plus the chopped tomatoes.

Set the pan over a medium heat until it reaches a gentle boil then turn down the heat. You can cover the pan or use a splatterguard (it is very messy otherwise) and I usually leave them cooking for around an hour. Once everything is mushy turn off the heat and leave it to cool a little. Oily tomato can really burn the skin, so don't take the risk.
If using a food mill you'll want to do a couple of ladlefuls at a time, removing herbs as you go (bay leaf doesn't purée well). I employ YummyHubby's biceps to do this part of the recipe for me.

Now, the optional extra step. Pour the purée back in the pan over a low heat, cover with a splatterguard and leave it to reduce for 20 minutes. It'll become super strong and concentrated, and you'll have slightly less left over for bottling, but it's totally worth it.
Taste it. I find it usually needs about a teaspoon each of salt and sugar, but it's personal preference. You can always do this when you come to use it, according to how you're serving it. And that's it. Done.
Once cool, pour into clean glass jars and keep in the fridge for up to a week. You can freeze it, too, though it's never quite the same as fresh.
If you're bottling to keep the sauce for months you need to do it properly. Basically you put the jars in hot water and boil them for 35 minutes to create a vacuum. But there's more to it than that and it'd be a shame to let your hard work go to waste by getting a step wrong. Look it up and follow the instructions and you'll have tomate frito for months to come.
how to eat it
There's the Italian way, piled up on freshly cooked pasta. While the pasta is draining, melt some butter along with salt and pepper in the pan, pour the pasta back in and stir through. Add a generous sprinkle of grated parmesan then turn out onto plates. Ladle the sauce over the top, garnish with basil leaves and you're done.

And there's the Spanish way. A favourite with egg-loving YummyHubby is easy-peasy Arroz a la Cubana. (I don't know if this is imported to Spain from Cuba or if the Spaniards made it up. Either way, super cheap and tasty.)
Sauté a little onion and garlic in a drop of oil, then add rice, continue to cook a little while, add water and a pinch of salt and simmer until done (you can add a handful of frozen peas along with the rice which totally works in this dish). Drain if needed and put into moulds (I use teacups) to set a little. Fry a couple of eggs and a plantain or banana. Turn out the rice onto the plate, add eggs and plantain. Pour a ladleful of tomate frito over the rice. Y está.
¡Que aproveche!
*If you don't have a pasapuré then you can roughly purée the sauce in a blender or food mixer and sieve it. Far too much faff for me. And you can get a decent pasapurés in this country for under £30 (as I discovered when my old faithful from Spain became too warped to work any more).
If you're new to them, the trick is to set them on top of a pan or a bowl which fits snugly, so that they stay stable. The one I have has legs which sit on the pan edge and it's very sturdy. My sis has a fabulous one with a collar all the way around making it super easy to use, it never wobbles. But they're quite pricey. It's a worthwhile investment if you make this, or gazpacho, often. Otherwise, meh, use a blender.