Bring a pot of water to a boil, adding generous salt.

Grate the parmesan into a bowl and zest and juice the lemon, setting aside separately.

Add the spaghetti to the boiling water and cook until almost al dente.

In a saucepan over medium heat, add olive oil, butter, and garlic. Sauté until garlic is fragrant and golden.

Pour in the heavy cream and bring to a simmer while whisking. Stir in ¾ of the lemon zest.

When the pasta reaches al dente, reserve 1 cup of pasta water, then drain the spaghetti.

Toss the spaghetti into the saucepan with the sauce, adding a ¼ cup of reserved pasta water and half of the grated parmesan. Stir with tongs.

Pour in the lemon juice, add remaining parmesan, and adjust sauce consistency with more pasta water or parmesan if needed.

Taste and add kosher salt as necessary.

Once the sauce has thickened slightly, divide the pasta into two bowls.

Garnish each serving with remaining lemon zest, extra parmesan, black pepper, and optional red chili flakes.