Heat the olive oil in a saucepan and gently sweat the onions for 5 minutes until they begin to soften. Add the garlic and continue to cook for a further 3 minutes.
Add the tomato puree to the saucepan and cook for a minute before stirring in the tinned tomatoes, chilli flakes, salt and pepper. Simmer gently for 20-25 minutes until sauce begins to thicken. This cooking helps to improve the flavour. If it gets too thick add a splash of water.
Some brands of chopped tomatoes can be quite tart even after cooking. If this is the case stir in 1 tsp of sugar to the sauce.
When nearly ready to serve cook the pasta, drain then stir through the sauce. Serve with pesto, rocket, parmesan and a slab of sourdough.