Heat your Dutch oven over medium-high heat. Add the sliced kielbasa and cook, stirring occasionally, until the edges caramelize and render their fat, about 6-8 minutes. You want deep golden-brown spots - this builds the flavor base. Transfer to a plate, leaving the rendered fat in the pot.

Add the penne and chicken broth to the pot. Bring to a boil, then reduce to a steady simmer. Cover and cook, stirring once halfway through, until the pasta is just al dente and most of the liquid is absorbed, about 12-14 minutes. There should still be a thin layer of starchy broth at the bottom - this is your sauce foundation.

Reduce heat to medium-low. Push the pasta to one side, add the butter to the exposed liquid, and once melted, stir in the garlic. Cook until fragrant, about 30 seconds, then mix everything together. Pour in the heavy cream and simmer gently for 2 minutes until slightly thickened.

Remove from heat. Stir in the Parmesan, black pepper, and crushed red pepper if using. The residual heat will melt the cheese into a silky sauce. Fold the browned kielbasa back in, letting it warm through for a minute. If the sauce seems too thick, splash in a little more broth or cream.

Taste and adjust seasoning - the kielbasa and cheese add salt, so you likely won't need much. Serve immediately in shallow bowls, topped with extra Parmesan and fresh parsley. The sauce tightens as it sits, so serve right away.