Bring a large pot of generously salted water to a boil. Cook the fettuccine pasta according to package instructions until al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta and set aside.

In a large skillet, heat the olive oil and butter over medium-high heat until the butter is melted and bubbling.

Add the sliced baby bella mushrooms in an even layer. Let them cook completely undisturbed for 4 to 5 minutes to develop a deep brown sear. Stir and cook for an additional 2 minutes until tender.

Reduce the heat to medium. Add the minced garlic and chopped fresh rosemary to the skillet, stirring constantly for 1 minute until fragrant.

Pour the heavy cream into the skillet, stirring to scrape up any browned bits from the bottom of the pan. Bring to a gentle simmer and cook for 2 to 3 minutes until the sauce begins to thicken slightly.

Stir in the freshly grated Parmesan cheese and black pepper. Continue stirring until the cheese is completely melted and the sauce is smooth and velvety.

Add the cooked fettuccine to the skillet, tossing gently to coat the noodles evenly. If the sauce is too thick, splash in 1 to 2 tablespoons of the reserved pasta water at a time until the desired consistency is reached.

Remove from heat, garnish with freshly chopped parsley, and serve immediately.