In a pot, cook tomatoes with 1 tsp of salt, cover and cook on low heat for 20 mins or so. The tomatoes will cook in their own water.
Heat a pan and add a drizzle of olive oil. Add the onion and garlic and cook on medium heat for a minute or so, being careful not to burn the garlic.
Add chilli flakes, cooked tomatoes, a sprig of rosemary and season lightly with salt and pepper. Cook for 20 - 25 minutes low heat, stirring occasionally, until the sauce has thickened.
Cool completely and blend lightly
Cook the pasta in a large pot of boiling salted water, until al dente. Drain, reserving 1 cup of pasta water.
In the same pan in which you cooked the tomatoes, transfer the blended sauce and heat lightly. Add some pasta water to adjust consistency. Toss in the pasta.
Remove the pan from the heat, add grated parmesan, shredded basil leaves and crushed black pepper.
Toss once again and serve.