Melt 1 tablespoon of butter in a medium pot over medium heat. Add the mushrooms, salt, and pepper and cook until the mushrooms are softened. Remove from the pot.
2. Step
Add the remaining 2 tablespoons of butter and the flour to the pot. Whisk until the flour is toasted, approximately 2 minutes.
3. Step
Add the milk while whisking continuously to avoid lumps. Cook until the sauce is thickened.
4. Step
Whisk in the Parmesan and cheddar cheeses and season with salt and pepper.
5. Step
Add the cooked pasta and mushrooms. Mix until the sauce coats the pasta well.
