Sierra Empanada

Name: Sierra Empanada
Ingredients: 1 onion, cut in quarters, plus 2 1/2 pounds onions, diced; 1 tablespoon plus 3 1/2 teaspoons salt; 2 1/2 pounds boneless, skinless chicken thighs; 1/2 cup olive oil, plus more for oiling the sheet pan; 1/3 cup chipotle peppers; 1 tablespoon garlic, minced; 1 cup crushed tomatoes; 1 teaspoon black pepper; 2 tablespoons chopped fresh chives; Empanada Dough, recipe follows; Egg yolk, for glaze, optional; 3 cups all-purpose flour; 1/2 teaspoon salt; 6 ounces unsalted butter; 1 egg