Bring a large pot of salted water to a boil. Add the pasta and cook until just shy of al dente.
Reserve 1 cup of the starchy pasta water and drain.
While the pasta cooks, warm olive oil and 2 tbsp butter in a wide skillet over medium heat. Add the onion and a pinch of salt. Cook, stirring, until soft and translucent, about 5–6 minutes.
Add the garlic and red pepper flakes.
Cook 30–60 seconds until fragrant—don’t let the garlic brown.
Pour in the crushed tomatoes. Season with 1 tsp salt, a few grinds of pepper, and the sugar if using. Simmer gently for 8–10 minutes, stirring occasionally.
Reduce the heat to low and stir in the cream.
Simmer 2–3 minutes to meld. Taste and adjust salt and pepper.
Add the drained pasta to the skillet with 1/3–1/2 cup reserved pasta water. Toss over medium heat until the sauce looks glossy and clings.
Add more pasta water as needed.
Stir in Parmesan until melted and smooth. Off the heat, fold in the basil. For extra silkiness, finish with a small knob of butter and a pinch of lemon zest.
Serve immediately with more Parmesan and freshly cracked black pepper.