
1 (17.3-oz.) package frozen puff pastry sheets, thawed (2 sheets)
1½ pounds boneless, skinless chicken breasts, cut into 1-inch pieces
1 teaspoon salt
1 teaspoon freshly ground black pepper
4 tablespoons unsalted butter, divided
1 small onion, chopped
4 cups sliced carrots and/or bite-size broccoli or cauliflower florets
2 cloves garlic, minced
1 recipe warm Classic Béchamel
2 cups shredded white Cheddar cheese