Step 1: Prepare the Whiskey Glazed Mushrooms. In a medium skillet, heat 1 tbsp olive oil over medium-high heat. Add sliced mushrooms and cook until softened and browned, about 5-7 minutes.
Step 2: Deglaze the pan by carefully adding the whiskey. Allow it to simmer for 1 minute to cook off the alcohol. Stir in brown sugar, Worcestershire sauce, and Dijon mustard. Cook until the glaze thickens slightly and coats the mushrooms, about 3-5 minutes. Season with salt and pepper to taste. Remove from heat and set aside.
Step 3: Prepare the Garlic Butter Drizzle. In a small bowl, combine 2 tbsp melted butter and minced garlic. Stir well. Set aside.
Step 4: Form the Beef Patties. Divide the ground chuck into 4 loosely packed balls (about 1/4 lb each). Do not compress them.
Step 5: Heat a large cast-iron skillet or griddle over medium-high heat until smoking. Add the burger balls to the hot surface, spacing them out.
Step 6: Smash the burgers. Using a sturdy, flat spatula or burger press, firmly press each ball down into a thin patty, about 1/4-inch thick. Hold for 5-10 seconds to create good contact.
Step 7: Season generously with salt and freshly ground black pepper immediately after smashing. Cook undisturbed for 2-3 minutes until a deep brown, crispy crust forms.
Step 8: While burgers cook, lightly butter the cut sides of the burger buns and toast them on a separate pan or on the same griddle during the last minute of burger cooking until golden brown.
Step 9: Flip the patties. Immediately place a slice of Swiss cheese on top of each patty. Cook for another 1-2 minutes until the cheese is melted and the burger is cooked through.
Step 10: Assemble the burgers. Place a cheesy patty on the bottom half of each toasted bun. Top generously with the whiskey-glazed mushrooms. Drizzle each burger with the garlic butter. Garnish with chopped parsley, if desired, and top with the other half of the bun. Serve immediately.