Tuna Noodle Casserole With Asparagus

Name: Tuna Noodle Casserole With Asparagus
Ingredients: 1 stick (1/2 cup) unsalted butter, plus more for the baking dish; 12 ounces wide egg noodles; 1 pound (1 bunch) asparagus, trimmed and cut into 1-inch pieces; 1 cup frozen peas; 1 cup panko bread crumbs; kosher salt and black pepper; 1/2 small onion, finely chopped; 1/3 cup all-purpose flour; 5 cups whole milk; 8 ounces Cheddar, grated (about 2 cups); 2 5-ounce cans tuna, drained; 1 tablespoon chopped fresh tarragon or 1 teaspoon dried tarragon