Add the sliced mushrooms to the pan. Cook them for about 5-7 minutes, stirring occasionally, until they release their moisture and become golden brown.
Combine Everything:
Once the mushrooms are cooked, add the butter to the pan and let it melt.
Toss in the cooked spaghetti and mix everything well. If the pasta seems dry, add a bit of the reserved pasta water to create a light sauce.
Season with salt, black pepper, and red pepper flakes (if using).
Serve:
Remove from heat and stir in the lemon juice (if using) and fresh parsley.
Serve immediately, topped with grated Parmesan cheese.
Tips:
You can add other ingredients like spinach, cherry tomatoes, or even a protein like grilled chicken or shrimp for added variety.
For a vegan version, replace butter with vegan butter and skip the Parmesan or use a vegan cheese alternative.