Burgers: In a large bowl, gently mix together ground chuck, ground sirloin, Stubb's Steak Spice Rub and Stubb's Texas Steakhouse Marinade. Refrigerate ground beef mixture for 20-30 minutes.
Meanwhile, preheat grill to high heat. Once ground meat is well chilled, wet hands with cold water and divide into six equal portions. Be careful not to over work the meat, and shape beef into ¾-inch thick patties. Using your thumb, press a small dimple in the center of the formed burger. Season prepared burgers with salt and black pepper. Transfer burgers to preheated grill. Cook burgers for 4-5 minutes per side, or until cooked through.
Mashed Potatoes: Bring a large pot of water to a boil. Carefully add peeled and diced potatoes to boiling water. Cook until the potatoes are tender, about 12 minutes. Drain water and return cooked potatoes to the pot.
Add milk and butter to pot with potatoes. Using a potato masher or electric mixer, mash the potatoes until desired consistency is reach. Season to taste with salt and black pepper.
Mushroom Gravy: Melt butter in a medium sized pot over medium heat. Add onions and cook, stirring occasionally, for 10-12 minutes or until onions start to caramelize. Stir in mushrooms and cook for 3-4 minutes. Stir in garlic and cook for 30 to 60 seconds.
Sprinkle flour over cooked vegetables, and stir to combine. Cook for 2-3 minutes, stirring constantly. Add 1 cup of beef stock, stirring constantly until fully incorporated. Stir in remaining stock and milk. Season gravy, to taste, with Stubb's Steak Spice Rub, salt and black pepper. Reduce heat to medium-low, and simmer until thickened, about 10-15 minutes, stirring often.
To assemble: Place cooked burger on the bottom toasted bun. Top burgers with approximately 1/3 cup mashed potatoes. Top mashed potatoes with 2-3 tablespoons prepared gravy. Top gravy covered potatoes with remaining bun. Serve immediately.