My cheat’s method: tin of tomato purée, dollop of ketchup, twist of black pepper and some dried basil and oregano. Or just the tomatoes if it’s going on top of home-made garlic butter which already contains the herbs.
Normal method: fine chop onions and a clove of garlic, fry gently until very soft, then add a tin of tomatoes and simmer until it’s reduced and thickened and tastes sweet rather than acidic. Then coarsely blend, season as above.
I’ve got too much time on my hands method: halve a load of fresh vine tomatoes, chop an onion and a red pepper into lumps. Chuck in a baking tray with a couple of cloves of garlic, drizzle with olive oil and then stir to coat, scatter fresh basil about and bang in the oven until it’s starting to char. Let it cool, chuck it in the blender adding veg stock to get the consistency right as needed.