How to Make It
Preheat oven to 375°F. Heat 1 tablespoon of the oil in a large skillet over medium-high. Add mushrooms, onion, and bell peppers; cook, stirring constantly, until veggies begin to soften, about 3 minutes. Add garlic powder, salt, and pepper; cook, stirring often, 2 minutes.
Add butter to veggie mixture; cook until butter is melted. Sprinkle in flour; cook, stirring constantly, 1 to 2 minutes. Whisk in milk, Worcestershire, and dry mustard until smooth.
Reduce heat to medium-low, and stir in Velveeta until smooth. Remove from heat, and stir in cooked pasta, roast beef, and 1 1/4 cups provolone until combined.
Pour mixture into an 11- x 7-inch baking dish lightly coated with cooking spray; top with remaining 3/4 cup cheese. Stir together panko and remaining 1 teaspoon oil in a small bowl. Sprinkle breadcrumb mixture evenly over pasta mixture, and bake in preheated oven until golden and bubbly, 20 to 25 minutes. Sprinkle with parsley, if desired.