Bring a medium saucepan of water to a boil. Add the pasta and cook for 5–7 minutes until tender. Drain and set aside.
In a 10-inch skillet, heat the olive oil over medium heat. Add the onions and cook for 2 minutes, stirring occasionally.
Stir in the garlic and red pepper flakes. Cook for 1 minute. (Reduce or skip the red pepper flakes for a milder sauce.)
Pour in the vodka and simmer for 2 minutes, stirring occasionally.
Stir in the diced tomatoes with some juice. Season with salt and pepper. Bring to a gentle boil, then reduce heat and simmer for 5 minutes.
Add the heavy cream and simmer for 2–3 minutes until the sauce thickens. Stir in the Parmesan.
Add the cooked pasta and toss to coat in the sauce. Top with extra Parmesan and serve.
Notes
Cook Pasta Al Dente: Boil the pasta until tender but still slightly firm, it will finish softening in the sauce.
Use a Skillet with Room: A 10-inch skillet works best so the sauce can reduce evenly and coat the pasta well.
Simmer the Sauce: Let the sauce simmer until it slightly thickens. Stir often to prevent sticking.
Taste and Season: Always taste before serving. Add salt if needed to balance the creaminess.
Add Parmesan Last: Stir in Parmesan at the end for better melting and flavor.
For Extra Richness: Stir in 1/2 tablespoon of butter after the sauce has thickened.
Skip the Vodka? Replace with water and a squeeze of lemon to maintain the brightness without the alcohol.
Note on Canned Diced Tomatoes: Use 1 cup (218g/7.7oz) of diced tomatoes with some juice—about half of a 14.5-ounce can. Measure by filling a 1-cup measuring cup with both tomatoes and juice. If needed, you can substitute with 1 cup of canned whole tomatoes (crushed by hand) or 1 cup of canned crushed tomatoes.