Warm the olive oil in a 12-inch sauté pan over medium-low heat, then add the onion and garlic; stir occasionally for 2 to 3 minutes. The onion should look translucent and smell sweet and fragrant, with no browning at the edges.

Stir in the diced tomatoes, tomato sauce, Italian seasoning, dried basil, sugar, salt, crushed red pepper if using, and black pepper; then fold in the heavy cream and Parmesan and cook for 1 to 2 minutes. Stop when the sauce is hot, softly bubbly, and pale coral pink with melted Parmesan throughout.

Toss the sauce with cooked pasta and finish with fresh basil if desired. The sauce should cling to the noodles; add a small splash of reserved pasta water if it tightens too much.