Green Chile–Chicken Enchiladas

Name: Green Chile–Chicken Enchiladas
Ingredients: 4 skin-on, bone-in chicken breast halves (4 1/2 pounds); 1 quart chicken stock or low-sodium broth; Kosher salt; Pepper; 6 tablespoons unsalted butter, plus more for greasing; 1/3 cup all-purpose flour; 1 1/2 cups whole milk; 18 corn tortillas; 12 ounces sharp white cheddar cheese, shredded (3 cups); 12 ounces jarred or canned chopped roasted Hatch chiles; 1/2 cup minced white onion; Cilantro leaves and thinly sliced jalapeño, for garnish