Set a big pot of salted water to boil on high to cook pasta.
While pasta is cooking, make the sauce.
Slice a thin round off the bottom of each tomato. Starting at cut end, grate tomatoes on the largest holes of a grater over a medium bowl until all that’s left is the flattened tomato skin and stem; discard. Take care while grating, as it gets slippery from the tomato juice. Don’t rush while you grate.
In a large skillet heat olive oil over medium-high heat. Add chopped onions, saute 2-3 minutes, until tender. Turn heat to medium. Add minced garlic. Saute a few minutes until garlic is fragrant and golden.
Add rosemary and cook, tossing, just until fragrant and starting to brown around edges, about 1 minute. Reduce heat to medium-low, add tomatoes, and bring to a simmer. Cook, stirring often, until mixture is slightly thickened, 5–10 minutes. Remove from heat, add butter, and stir until melted.