Cook your pasta as per packet instructions. And drain roughly, leaving a little bit of pasta water in the pan.
With a sharp knife finely dice the onion, and roughly chop the basil leaves.
Add the onion to a saucepan on a medium heat with the olive oil. Cook until the onions are JUST opaque.
Add the tomatoes and salt and pepper. Heat until the sauce JUST starts to bubble, them remove from the heat.
While off the heat, add the chopped basil and crush the garlic cloves into the sauce. Mix well
Put back on the heat until it JUST starts to bubble again, remove quickly and add to the cooked pasta.