Preheat oven to 375 degrees F.
Heat butter in a large sauce pan over medium-high heat. Add onions, cook for 6-7 minutes or until translucent. Add celery and cook for 3-4 minutes. Add garlic and cook for an additional 30 seconds to 1 minute.
Stir in flour, stirring constantly until a thick, golden roux is formed, about 2-3 minutes. Stirring constantly, stir in turkey broth and milk. Simmer for 4-5 minutes, stirring occasionally, or until gravy is thickened. Stir in Stubb’s Chicken Spice Rub, thyme, sage, turkey and mixed vegetables. Simmer, stirring occasionally, for 5-6 minutes.
Pour prepared Shepherd’s Pie filling into a 9x13-inch baking dish. Spread prepared mashed potatoes evenly over the meat and vegetable filling. Bake in preheated 375 degree F oven for 20-25 minutes or until potatoes start to brown and casserole is heated through.