Salsa di pomodoro (basic tomato sauce) is the single most important sauce in Italian cooking—a quick, simple preparation of San Marzano or other quality canned tomatoes cooked briefly with olive oil, garlic (or onion), basil, and salt that, when properly made, transcends its humble ingredients to become a bright, balanced, deeply satisfying sauce that is the mother of dozens of Italian preparations. The essential principle is restraint: good tomato sauce is made by cooking tomatoes as little as possible—just long enough to meld the flavours and thicken slightly, but not so long that the bright, fresh tomato flavour dulls into the heavy, overcooked taste that plagues bad Italian-American red sauce. Marcella Hazan's iconic version uses only canned tomatoes, butter, and a halved onion (removed before serving)—nothing else. The simplest version: olive oil in a pan, a clove of garlic (whole, gently pressed, removed when golden), a can of San Marzano tomatoes (crushed by hand), salt, and fresh basil leaves—simmered for 15-20 minutes until slightly thickened but still bright and fresh-tasting. That's it. The tomatoes must be excellent quality—San Marzano DOP from the Agro Sarzanese-Nocerino in Campania is the gold standard, though other high-quality Italian tinned tomatoes (Mutti, Strianese, Cento) are excellent. The sauce should taste of tomato, not of cooking. It should be bright red, not dark. It should have sweetness from the tomato itself, not from added sugar. When you achieve this, you understand why Italian cooking prizes ingredients over technique.
Quality canned tomatoes (San Marzano DOP ideal). Olive oil + garlic (or onion or butter). Fresh basil. Cook briefly—15-20 minutes maximum. The sauce should taste of fresh tomato, not of long cooking. No sugar. Simplicity is the point.
Crush whole canned tomatoes by hand rather than using pre-crushed—the texture is better and you remove any hard core pieces. A teaspoon of butter whisked in at the end adds a silky richness (Marcella's secret). If tomatoes are acidic, a tiny pinch of baking soda neutralizes acidity better than sugar. The basil should be added at the end—not at the beginning—for the freshest flavour. A splash of pasta water added to the sauce just before tossing with pasta creates the perfect consistency.
Cooking too long (the sauce loses freshness and turns dark). Adding sugar (good tomatoes don't need it). Using poor-quality tinned tomatoes (the tomato IS the sauce). Adding too many ingredients (keep it simple). Over-browning the garlic (should be gently golden, not dark).
Marcella Hazan, Essentials of Classic Italian Cooking; Pellegrino Artusi, Science in the Kitchen