Pulled Pork Mac and Cheese

Name: Pulled Pork Mac and Cheese
Ingredients: Cooking spray; 1 pound uncooked large elbow pasta; 0.5 cup unsalted butter, divided; 1.5 cups finely chopped sweet onion (about 1 medium onion); 0.25 cup all-purpose flour; 3.5 cups whole milk; 2 teaspoons Dijon mustard; 1 teaspoon garlic powder; 0.5 teaspoon black pepper; 8 ounces extra-sharp Cheddar cheese, shredded (about 2 cups); 8 ounces Colby-Jack cheese, shredded (about 2 cups); 2.5 cups shredded smoked pork (sauceless); 1 cup panko; 0.25 cup sliced scallions (about 2 medium scallions); Barbecue sauce