Preheat oven to 400°F.
Place the bread halves onto a baking sheet and toast in the oven until golden brown, about 5 minutes.
In a small saucepan, heat EVOO over medium heat. When oil starts to ripple, add in bell peppers, onions and garlic; cook until soft. Add butter to veggies then sprinkle in flour to create a roux. Stir for about 2 minutes and slowly pour milk in, stirring constantly.
When sauce is thickened, remove from heat. Add half of the cheese and stir gently until it melts. Let sauce cool about 10 minutes then pour over toasted French bread crusts.
Layer roast beef across pizza bread and top with remaining cheese and mushrooms. Bake 10 minutes or until cheese starts to bubble and brown.
Serve immediately, garnished with fresh parsley.