Combine onion and next 8 ingredients in large Dutch oven or large heavy-bottomed pot over high heat and bring to boil. Cover; reduce heat and bring just to simmer. Simmer 30 minutes. Strain liquid into large bowl, discarding solids. Set aside to cool.
Mix mayonnaise, horseradish and grainy mustard. Refrigerate.
Add olive oil to large skillet; place over medium heat. Add onion and cook until transparent. Add button and baby bella slices; stir, and cook in single layer for about 10 minutes or until golden brown, stirring only occasionally. Add butter, garlic, thyme and salt to skillet; heat until butter is melted, stirring to coat. Add wine; cook until liquid is thick and almost evaporated. Remove from heat.
Heat large skillet over medium-low heat. Spread cut sides of each roll with mayonnaise mixture. Refrigerate any leftovers for another use. Place gouda cheese slice on bottom half of each roll, followed by 3 bacon slices each. Trim to fit sandwich if necessary. Top with spoonful of the mushrooms and provolone cheese slice. Place roll tops over to form sandwiches. Brush roll tops and bottoms with melted butter. Place sandwiches in skillet; cook until crispy and golden brown on both sides, flipping once, about 5 minutes. Remove from pan to platter or plate.
If needed, reheat au jus in pan over high heat. Dunk sandwiches in au jus: with slotted spatula, lift 1 sandwich at a time and carefully dunk it in the reserved au jus sauce, 5-10 seconds, using large spoon to quickly spoon sauce over sandwich top if necessary. Repeat with remaining sandwiches. Serve immediately with additional au jus sauce in a small cup for dipping on the side.
- 3/4 cup mayonnaise
- 1 tablespoon prepared horseradish
- 2 tablespoons grainy mustard
- 3 tablespoons extra virgin olive oil
- 1/2 medium onion, thinly sliced
- 8 ounces fresh button mushrooms, sliced
- 8 ounces fresh baby bella mushrooms, sliced
- 2 tablespoons butter
- 1 clove garlic, finely minced
- 2 teaspoons chopped fresh thyme leaves
- 1 teaspoon kosher salt
- 1/4 cup Cabernet Sauvignon wine
- 4 large square pretzel rolls, split (can substitute ciabatta rolls)
- 4 slices (1-ounce each) Wisconsin Gouda cheese, cut to fit rolls, at room temperature
- 12 slices cooked thick-cut hickory smoked bacon
- 4 slices (1-ounce each) Wisconsin Provolone cheese, cut to fit rolls, at room temperature
- 4 tablespoons butter, melted
For Au Jus:
- 1/2 medium onion, roughly chopped
- 2 cloves garlic, roughly chopped
- 3 tablespoons Worcestershire sauce
- 1/2 cup Cabernet Sauvignon wine
- 6 cups beef stock
- 2 tablespoons fresh thyme leaves, chopped
- 2 tablespoons fresh rosemary, chopped
- 1/2 teaspoon kosher salt
- 2 teaspoons coarsely ground black pepper