Mix ground turkey, bread crumbs, egg, onions, and mustard in large bowl. Shape into 4 patties, about 1/2-inch thick.

Melt butter in large skillet over low heat. Add patties and cook, turning once to brown other side. Cook until golden brown outside and white inside, about 10 minutes. Remove from skillet and place onto plate.

Add chicken broth to skillet, and boil over high heat until slightly thickened, about 1 to 2 minutes. Pour sauce over patties. Serve on buns with lettuce leaf.