This gets used so much in my cooking that it seemed sensible to tell you how I make it! It's not rocket science, but it is tasty.
Basic Tomato Sauce
1 onion, finely sliced or chopped
1 carton or tin of chopped tomatoes
1 carton passata
some dried herbs - roughly a teaspoon
some garlic - 2 crushed cloves, a tsp of ready chopped/paste or tsp of granules
2 heaped tbsp tomato puree
Heat a splash of olive oil in a pan, chuck in the onions and cook over a medium heat until soft and caramel brown.
Add everything else. Stir.
Simmer for 15 minutes until thick.