Place oil and butter over high heat. Once butter has melted add in onions and garlic. Mix through and saute for a few minutes then reduce heat so the onions cook gradually.
Add in fresh diced tomatoes. Cook, stirring occasionally. You want the tomatoes to cook down a bit and blend in with the butter to make a thick sauce consistency. Mix through your basil and season with salt and pepper.
Add tinned tomatoes. Mix through, bring to the boil and then reduce and simmer for 10 minutes. You can then leave the sauce as is or puree with a stick mixer for a smooth sauce.