Bring a large pot of water to a rolling boil and add 1 tbsp kosher salt per 4 liters (a well-salted pot seasons pasta from the inside). This is essential because under-salted pasta cannot be fully corrected by sauce later. Keep the pot covered until it boils to speed the process.

Prepare the base: Finely chop the onion and mince 4 of the garlic cloves; reserve 2 cloves thinly sliced for the breadcrumb topping. Heat 2 tbsp extra-virgin olive oil plus 1 tbsp butter in a wide sauté pan over medium heat. Add the onion and a pinch of salt and sweat gently, stirring occasionally, until translucent (about 6–8 minutes). Sweating at moderate heat extracts sweetness without browning—key for a clean tomato flavor.

Add the minced garlic and red pepper flakes to the onions and cook for 30–45 seconds until fragrant. Don't let garlic brown—brief cooking blooms its aromatic oils without developing bitterness.

Stir in the tomato paste and cook for 45 seconds to 1 minute to 'toast' it. Toasting deepens the tomato paste's flavor and removes any raw metallic edge.

Pour in the crushed tomatoes, add the sugar and 1/2 tsp kosher salt, and bring to a gentle simmer. Simmer, uncovered, for 12–15 minutes, stirring occasionally, until the sauce thickens slightly and brightens (you want concentrated tomato flavor but still enough moisture to finish the pasta). Taste and adjust salt and pepper.

While the sauce simmers, cook the spaghetti in the boiling, salted water until just shy of al dente (about 1–2 minutes less than the package time). Reserve 1 cup (240 ml) of pasta cooking water before draining. Cooking the pasta slightly under ensures it finishes cooking in the sauce and absorbs flavor without turning mushy.

Make the crispy garlic breadcrumbs: In a small skillet, heat 1 tbsp olive oil over medium heat. Add the thinly sliced garlic and cook 30–45 seconds until fragrant. Add the panko and a pinch of salt and toast, stirring, until the crumbs are golden and evenly colored (about 3–4 minutes). Finish with lemon zest and 1 tbsp grated Parmesan. Browning the crumbs gives a nutty crunch and the quick garlic crisp adds an aromatic pop—add this at the end so they stay crunchy.

Finish the sauce: Remove the pan from heat, tear in most of the basil (reserve a few leaves for garnish), stir in the remaining 1 tbsp butter and 1/2 cup grated Parmesan to enrich the sauce and provide silkiness. The residual heat will melt cheese and butter into a glossy emulsion—this technique (mantecatura) binds sauce to pasta.

Combine pasta and sauce: Return the sauce to low heat, add the spaghetti and toss vigorously, adding reserved pasta water a little at a time (start with 1/4 cup) until the sauce clings to the pasta. Continue tossing for 1–2 minutes so the starches in the water help emulsify the sauce—this helps the sauce adhere and creates a silky finish.

Plate immediately: Divide among four warm plates, scatter the crispy garlic breadcrumbs generously over each portion, finish with the remaining Parmesan, torn basil leaves, and a final drizzle of olive oil and freshly ground black pepper. The contrast between silky pasta and crunchy topping is the signature of this dish—serve right away to keep textures.

Optional finishing touch: For extra freshness, squeeze a little lemon juice over each plate (about 1/8 tsp per serving) just before serving to lift the tomato flavor and balance the richness.