Slice onion finely.
Heat onion gently with butter in a sauce pot until just beginning to caramelize.
Add diced tomato. Simmer gently for about 15 minutes.
Get your pasta cooking.
Add vinegar to marinara. Continue simmering.
When the pasta is nearly cooked to your liking, strain it and transfer into the sauce.
Continue cooking pasta gently in sauce, just until done.
Season, serve and consume. I tossed some arugula and olive oil on mine, but that's all just decoration really.