Cook the spaghetti according to package directions. Reserve 1 cup of pasta water then drain the spaghetti.: A fragrant cloud of boiling water and the sound of lively bubbles signal that the pasta is cooking at a good pace. The starchy pasta water you reserve is liquid gold, because it helps emulsion and bind the sauce to the strands, creating a glossy finish. One common mistake is draining everything and discarding the water, which makes the sauce thin and separate. If the pasta feels too firm or too soft, adjust the next batch by shortening or lengthening the cooking time slightly.
Meanwhile, add the butter and oil to a large Dutch oven and heat over medium heat. Add the onion and cook for 5 minutes until the onion softens. Add the garlic and cook for another 30 seconds until fragrant.: As the butter melts into the olive oil , you will notice a faint nutty aroma and a shimmering surface on the pan. The onion should become translucent and slightly glossy, releasing gentle sweetness. When you add the garlic , listen for a soft sizzle and breathe in the fragrant, toasty scent; do not let the garlic turn brown, because that bitterness will carry through the whole dish. A frequent error here is using too high heat, which causes quick browning instead of gentle softening.
Add the tomatoes, salt, pepper, sugar and red pepper flakes. Cook for about 5 minutes until the tomato starts to break down, stirring occasionally. Reduce heat to simmer and cook for 15 minutes. Taste for seasoning and adjust as necessary.: The tomatoes will first steam and then begin to collapse, releasing bright, slightly tart steam that fills the kitchen. Stirring occasionally helps them break down evenly, creating a mix of juicy bits and saucy liquid. Red pepper flakes will perfume the steam with a subtle warmth, while sugar tames sharp edges. As it simmers, the sauce will thicken and the flavors will concentrate. Avoid rushing this step with high heat, which can cause uneven cooking and a harsher tomato flavor.
Stir in the basil. Add the cooked spaghetti and half of the pasta water. Toss everything well to combine. Add more pasta water if needed.: When the basil hits the hot sauce, you will smell a fresh, peppery lift, a sign to move quickly so the herb stays vibrant. The reserved pasta water helps create a silky emulsion, and the strands should glisten and look coated rather than sitting in a puddle. If the sauce seems dry, add small splashes of water until the texture becomes glossy and clingy. A typical mistake is dumping all the water at once, which can make the sauce too loose; add gradually until satisfied.
Serve topped with Parmesan cheese and more basil.: The final presentation is the payoff, with steam rising and the sharp, nutty perfume of freshly grated Parmesan cheese melting into warm pasta. The contrast of warm noodles and cool, bright basil heightens the experience. Avoid leaving the pasta in the pot too long, because residual heat will continue to soften it and may lead to a mushy texture. Serve promptly so each bite keeps its intended tension and freshness.