Joanna Gaines' Chicken Pot Pie

Nonstick cooking spray, for the pan

salted butter

small white onion, finely chopped (optional)

carrots, peeled and cut into 1/2-inch dice

all-purpose flour

chicken broth, store-bought or homemade

condensed cream of chicken soup

shredded meat from 1 store-bought rotisserie chicken

frozen peas

Kosher salt and freshly ground black pepper

tubes refrigerated crescent rolls

Mashed potatoes, for serving