Start by boiling a pot of water until it is bubbling steadily. Add the fettuccine noodles to the pot. Cook the pasta to al dente according to package instructions, making sure to keep a close watch to avoid overcooking and ending up with mushy pasta.
Heat the olive oil in a large skillet over medium heat. Add the sliced baby bella mushrooms to the skillet. Sauté the mushrooms for a few minutes until they are golden brown and fragrant. Make sure not to overcrowd the pan to prevent them from steaming.
Add the salted butter to the skillet with the mushrooms and let it melt completely. Once melted, sprinkle in the all-purpose flour and stir constantly. This mixture will start to bubble and thicken, which is key for creating a creamy base. Avoid burning it; you want a nice, light color.
Gradually pour in the chicken broth while whisking continuously, followed by the heavy cream. Keep whisking until the mixture is smooth and velvety. If lumps start to form, continue whisking until everything is blended without separation.
Stir in the freshly grated parmesan cheese along with the garlic powder. Mix until the cheese fully melts into the sauce, making it rich and comforting. If the sauce seems too thick, add a splash of chicken broth or pasta water to reach your desired consistency.
Taste the sauce and add salt and black pepper as necessary. Stir thoroughly to ensure the spices are evenly incorporated. Remember to season gradually, as a little goes a long way.
Finally, toss the chopped spinach into the sauce and give it a final stir, allowing the spinach to wilt down and turn vibrant green. Serve the velvety mushroom Alfredo sauce over the fettuccine noodles and enjoy every creamy bite!