Bring a large pot of generously salted water to a boil. Cook the rigatoni until just al dente according to the package directions, reserve at least 1 cup of pasta water, and drain.

While the pasta cooks, heat the olive oil in a large skillet or Dutch oven over medium-high heat. Add the pancetta or bacon if using and cook for about 5 minutes, until browned and crisp; transfer it to a plate while leaving the rendered fat in the pan.

Add the sliced Brussels sprouts to the pan and season lightly with salt and pepper. Cook for 5 to 7 minutes, stirring occasionally, until browned at the edges and tender but not mushy.

Add the shallot, garlic, and red-pepper flakes. Cook for 1 to 2 minutes, stirring often, until the aromatics are fragrant but not darkened.

Pour in the white wine if using and simmer for 1 to 2 minutes, scraping up browned bits from the pan. Add the heavy cream and simmer gently for 3 to 4 minutes, until slightly thickened.

Add the drained rigatoni, Parmesan, lemon zest, lemon juice, and cooked pancetta or bacon. Toss thoroughly, adding reserved pasta water a few tablespoons at a time until the sauce is glossy and coats every tube of pasta.

Taste and adjust the salt, pepper, lemon juice, and red-pepper flakes. Serve immediately with additional Parmesan sprinkled over each portion.