Cheesesteak Panini

Name: Cheesesteak Panini
Ingredients: 6 tablespoons salted butter; 1 small red bell pepper, sliced; 1 small yellow bell pepper, sliced; 1/2 yellow onion, sliced; 1/2 teaspoon kosher salt; 8 ounces thinly sliced deli roast beef; Couple dashes of Worcestershire; 2 tablespoons whole-grain mustard; 4 slices thick-sliced bread (Texas toast); 4 slices provolone cheese; 1/4 cup Homemade Cheese Sauce, recipe follows; One 1-pound block processed cheese, such as Velveeta, cut into cubes; One 5-ounce can evaporated milk; 1 teaspoon dry mustard; 1 teaspoon white wine vinegar; Pinch kosher salt; 1 clove garlic