Mix steak rub by combining brown sugar with garlic, smoked paprika, 1 tablespoon salt, 1 tablespoon pepper and dry mustard.
Add ground beef to a bowl and mix in steak rub. Form into 6 patties with an indention in the center.
Prepare grill with a 300° fire with hickory or oak wood for smoke flavor.
Wash mushrooms and pat dry with paper towels. Thinly slice and toss with olive oil, 1/2 teaspoon salt and 1/2 teaspoon pepper. Place on a grill pan.
Add burgers to the grill over the direct fire and place mushrooms over indirect heat.
Cook burgers 8- 10 minutes on each side or until internal temperature reaches 165°.
Stir mushrooms occasionally.
Top burgers with Swiss cheese. Remove burgers and mushrooms from the grill.
Layer buns with onions strings, burger patties, mushrooms and your favorite condiments.