In a large pot, add butter and the chopped onions and bell peppers. Sauté till the onions are softened.
Then add flour and mix well, cook for a minute or two to remove the raw taste.
Add 1 cup milk, little by little, and stir on low flame. Continue to stir on low flame till a creamy consistency is attained.
Add the cheese and season with salt, pepper and oregano. Mix well to combine. If the sauce feels too thick, add more milk. Remove from heat and keep aside.
To finish, add the mushrooms and pasta to the white sauce and mix well to combine. To loosen up (if necessary); add a little bit of the reserved pasta water.
Serve warm with grated parmesan. Enjoy with a glass of chilled white.