Cook the Pasta: Fill a large pot with water and add a generous pinch of salt. Bring it to a boil, then add the spaghetti. Cook until just tender, following the package timing. Reserve a cup of pasta water before draining.
Start the Sauce Base: Heat olive oil in a wide pan over medium heat. Add the chopped onion and cook slowly until soft and translucent. Stir often so the onion does not brown.
Add Garlic: Stir in the minced garlic and cook for about thirty seconds. Keep it moving so it stays fragrant and does not burn.
Build the Tomato Sauce: Add the chopped tomatoes to the pan. Sprinkle in sugar, red pepper flakes, salt, and pepper. Stir well and let the sauce simmer gently for about fifteen minutes until slightly thickened.
Smooth the Flavor: Add the butter to the sauce and stir until melted. This step softens the tomato acidity and gives the sauce a rounded taste.
Combine Pasta and Sauce: Add the drained spaghetti directly into the sauce. Toss well so every strand gets coated. Add a splash of pasta water if the sauce feels too thick.
Finish and Serve: Remove from heat and fold in fresh basil. Serve hot with grated Parmesan on top.
