Bring a large pot of generously salted water to a boil. Pat the steak dry with paper towels and season all sides liberally with 1/2 tsp salt and 1/2 tsp black pepper.

Heat olive oil in a large skillet over medium-high heat until it shimmers. Carefully place the steak in the hot skillet and sear for 3-5 minutes per side for medium-rare, or until it reaches your desired doneness. Remove the steak from the skillet and set it on a cutting board to rest.

While the steak is cooking, add the pasta to the boiling water and cook according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water.

Reduce the heat under the skillet to medium. Add the butter and let it melt. Add the chopped shallot and cook for 2 minutes until softened. Add the sliced mushrooms, the remaining 1/2 tsp salt, and 1/2 tsp black pepper. Cook, stirring occasionally, for 5-7 minutes until the mushrooms are browned and have released their liquid.

Add the minced garlic and red pepper flakes (if using) to the skillet and cook for 1 minute until fragrant.

Pour in the white wine (or broth) to deglaze the pan, scraping up any browned bits from the bottom. Let it simmer for 1-2 minutes until the liquid has mostly evaporated.

Stir in the heavy cream and bring the sauce to a gentle simmer. Reduce the heat to low and slowly stir in the grated Parmesan cheese until the sauce is smooth and creamy. Do not let it boil after adding the cheese.

Add the fresh spinach to the sauce and stir until it wilts, about 1-2 minutes. Taste the sauce and adjust seasoning if needed.

Add the drained pasta to the skillet with the sauce. Toss well to coat everything. If the sauce is too thick, add a splash of the reserved pasta water until it reaches your desired consistency.

Slice the rested steak thinly against the grain. You can either stir the steak slices into the pasta or serve the pasta in bowls and arrange the steak on top.

Garnish with fresh parsley and extra Parmesan cheese before serving.