This recipe is adapted form from Marcella Hazan's Tomato Sauce With Onion & Butter. My tweaks were to brown the butter in the pan first before adding the tomatoes (whole, peeled, canned) and the onion. I finished by pureeing the sauce right in the pan after fishing out the onion using an immersion blender. Liked the smoother consistency, and the extra toasty notes from the brown butter. Subtle, but a good way to mellow the tomatoes acidity. —chez_mere
Over medium high heat, cook the butter until browned bits form on the bottom and it smells nice and toasty. Add the tomatoes, half of an onion, and a large pinch of salt. Stir, then reduce heat to maintain a gentle simmer.
Cook, uncovered, for 45 minutes stirring occasionally. Remove the onion from the pot (save for another use, or discard). Using an immersion blender, puree the sauce until you have just a few chunks remaining.
Serve over pasta, as a dipping sauce for bread, with meatballs, or whatever else makes you happy.