Grate onion into saucepan with a tbs of olive oil, and sweat over medium heat until liquid evaporates. Add salt, and a pinch of red pepper flakes, if desired. Finely slice garlic and add to pan.
Chop tomatoes into a mixture of larger chunks and 1/2 inch dice. Put tomatoes into pan with onions and garlic, and simmer on medium heat for 15-20 minutes.
Add salt and more red pepper flakes, if desired. Cook down tomatoes until desired consistency is reached. Purée part of mixture with immersion blender, then return to pan (optional).
Cook your choice of pasta (bucatini shown here) in salted water until just before al dente. Strain pasta and place into sauce to finish cooking. Add pasta water to loosen pasta and sauce if needed.