Empty tomatoes into sauce pan, cut onion in half and place cut side down into pan, slice up the butter and place it around the sauce, sprinkle a pinch of salt over everything. Bring to a simmer over medium-high heat.
Once the sauce is simmering, turn heat down to low and continue to simmer slowly, for about 45 minutes total. Stirring, and crushing up the tomatoes, occasionally.
After 45 minutes the sauce is ready. Using tongs, discard the onion (or repurpose however you like). You can leave the sauce as is, or blend it with an immersion blender, food processor, stand blender, etc., to your desired consistency.