Prepare Cheese Sauce; keep warm.
Heat oil in a large nonstick skillet over medium-high heat. Add bell peppers and onion; sauté 8 minutes or until tender. Stir in vinegar, sugar, oregano, and garlic powder.
Place the toast slices on a baking sheet. Top each toast slice with 3 ounces roast beef, 6 tablespoons Cheese Sauce, and 3/4 cup bell pepper mixture. Broil for 1 minute or until the sandwiches are thoroughly heated. Sprinkle the sandwiches with chopped parsley.