Meanwhile, heat butter in a large skillet over medium-high heat. Add onions and celery; sauté until tender, about 8 minutes. Add to bowl of cornbread cubes, along with herbs, fennel, and broth mixture; toss to coat. Taste and adjust seasonings, including salt and pepper to taste. Turn into a greased 13- by 9-inch baking pan and cover with cooking spray-coated foil; bake until golden brown, about 30 minutes. Remove foil from pan and broil to brown top if needed, just a couple of minutes; serve.