Cook the pasta according to package instructions in a large pot of salted boiling water until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.

In a large skillet over medium heat, add the bone marrow and olive oil. Cook for about 5 minutes, stirring occasionally, until the marrow is melted and bubbly.

Add the minced garlic and red pepper flakes (if using) to the skillet. Sauté for 1-2 minutes until fragrant, being careful not to burn the garlic.

Pour in the heavy cream and stir to combine. Let it simmer for about 2-3 minutes until slightly thickened.

Stir in the grated Parmesan cheese and mix until melted and smooth. If the sauce is too thick, add some reserved pasta water, a little at a time, until desired consistency is reached.

Add the cooked pasta to the skillet and toss to coat in the sauce. Season with salt and pepper to taste.

Serve immediately, garnished with fresh parsley.