Preheat oven to 500 degrees. Place crust on greased baking sheet. In a small saucepan heat olive oil over med heat, when oil starts to ripple add in bell peppers and onions. Cook until soft. Next, melt in butter and sprinkle in flour, to create a roux. Stir for about 2 minutes Slowly pour milk in, stirring constantly until very warm, Be sure not to let it come to a boil. You should start to see a bit of steam coming from the pan. When sauce is thickened, remove from heat. Add in Parmesan cheese and stir gently until it is melted. Let sauce cool about 10 mins and pour over crust. Layer roast beef across pizza and top with cheddar cheese and mushrooms(if desired). Bake 15-20 mins until edges of crust are golden brown and cheese starts to bubble.