Trim the stem end of the tomatoes and cut off any deep imperfections. Coarsely chop the tomatoes. Set aside with juices from cutting board.
Cook onion with a pinch of salt, olive oil and half the butter over low to moderate heat until it softens.
Once the onions are soft add the chopped tomatoes.
Add salt to taste and the remaining butter, and cook the tomatoes and onion together over a low simmer until the tomatoes break down and a sauce begins to form. Feel free to mash the tomatoes with a fork or potato masher as they soften.
Note the change in color from red to orange. This color change is a key indicator the the fat (either oil or butter or both) and the tomatoes are combining into a saucy suspension. No orange color, not enough fat to give you the desired mouth feel.
Continue cooking over at a lively simmer until sauce thickens enough to nap the pasta. Toss with cooked pasta.