Divide the ground beef into 6 patties, or 12 small patties if going the slider route. In a large skillet or dutch oven, cook the patties until browned on both sides and cooked completely through. After they're cooked, set them aside, and discard excess grease.

In the same skillet, melt 3 tablespoons of butter. Add in the mushrooms, onions, and garlic, and sauté until the mushrooms are tender. Add in the Worcestershire sauce, cream, broth, salt, and pepper. Bring the sauce to a boil, stirring constantly, and then stir in the cornstarch mixture. Once the sauce has thickened, remove it from the heat and set it aside.

Slice open the pretzel buns and spread some mayo over the top and bottoms of each bun. Place the cooked patties on the bottom of bun and add the pepper jack cheese on top. Spoon a generous amount of the mushroom sauce over the top of each patty and replace the top of the bun. Put the assembled burger in a greased 9 x 13 baking dish. Melt the 1/2 cup of butter for the topping and stir in the poppy seeds and Worcestershire sauce. Spoon the melted butter over the top of the sliders. Cover the dish tightly with foil and bake at 375 degrees for about 15 minutes. Serve hot.