Bring a large pot of water to a rolling boil and add a generous pinch of salt (about 1 tbsp). Use heavily salted water because the pasta will absorb the seasoning as it cooks; properly seasoned water is the foundation of well-flavored pasta.

Peel and finely dice the onion; mince four cloves of garlic, reserving two whole cloves (or thin slices) for the breadcrumbs. The diced onion will provide sweet savory depth to the sauce; reserved garlic will crisp and flavor the breadcrumb topping.

Heat a large sauté pan over medium heat and add 2 tbsp olive oil. When the oil shimmers, add the diced onion and a pinch of salt; sweat the onion for 6–8 minutes until soft and translucent but not browned. Sweating extracts moisture and sweetness without creating bitter burnt edges — you want a sweet base for the tomato sauce.

Add the minced garlic and red pepper flakes to the pan and cook for 30–45 seconds until fragrant. Cooking the garlic briefly at this stage blooms its aroma into the oil — don’t overcook it or it will turn bitter.

Pour in the crushed tomatoes, add the dried oregano, sugar, and 1 tsp salt. Bring to a gentle simmer, then lower the heat and let it cook uncovered for 12–15 minutes, stirring occasionally, until the sauce thickens and the raw tomato flavor cooks out. Simmering concentrates flavors and lets the acidity mellow — the sugar is optional but helps balance bright tomatoes.

While the sauce simmers, cook the spaghetti in the boiling salted water 1–2 minutes less than package instructions (about 8 minutes if the package says 10). Undercooking the pasta for now is intentional: finish cooking it in the sauce so the pasta absorbs flavor and achieves perfect al dente texture.

Reserve 1 cup (240 ml) of the pasta cooking water, then drain the spaghetti. Pasta water contains starch that will help emulsify and thicken the sauce when you finish the pasta together with the sauce.

Make the crispy garlic breadcrumbs: in a small skillet over medium heat, melt the butter with 2 tbsp olive oil. Add the reserved two cloves sliced thin and cook 30 seconds until fragrant, then add the breadcrumbs and a pinch of salt. Toast, stirring frequently, for 4–6 minutes until breadcrumbs are golden and crisp. The butter adds richness and helps the breadcrumbs brown evenly; keep them moving so they don’t burn.

If your sauce is too acidic or thin, adjust now: add up to 1/4 cup reserved pasta water to loosen, or simmer 2–3 more minutes to reduce. Taste and add remaining 1/2 tsp salt or black pepper as needed. Finishing the sauce by adjusting texture and seasoning ensures a balanced final plate.

Add the drained pasta directly into the sauce pan with the simmering tomatoes. Toss with 2–4 tbsp reserved pasta water and 1 tbsp olive oil, stirring or tossing over medium heat for 1–2 minutes until the pasta is just al dente and the sauce clings evenly. This technique — cooking the pasta a little in the sauce with starchy water — creates a glossy, cohesive coating rather than a separate noodle and sauce.

Stir in half the grated Parmesan (about 1/4 cup) and the chopped parsley (if using) off the heat to add saltiness and a silky finish; the residual heat melts the cheese without overcooking. If you prefer saucier pasta, add up to 2 more tbsp of pasta water.

Divide pasta among four bowls. Sprinkle each portion generously with the crispy garlic breadcrumbs and the remaining grated Parmesan. The breadcrumbs add textural contrast and savory toasty notes that lift the whole dish — don’t skip them.

Finish each plate with a drizzle of the remaining olive oil and a crack of black pepper. Serve immediately. Olive oil at the end heightens aroma and adds a glossy mouthfeel.