Grind dried mushrooms into a fine powder using a spice grinder or mortar and pestle.
In a bowl, combine ground beef, mushroom powder, Worcestershire sauce, salt, pepper, and red pepper flakes (if using). Lightly mix until just combined.
Shape the mixture into 4 oz (120g) patties, making a small indentation in the center to prevent puffing.
Heat a skillet or grill over medium-high heat. Add olive oil, then sear patties for 3-4 minutes per side until browned and cooked to desired doneness.
Add a slice of Swiss cheese to each patty during the last minute of cooking to melt.
In the same skillet, cook sliced onions and shiitake mushrooms over medium heat with a bit of olive oil, stirring occasionally until tender (5-7 minutes). Season with salt and pepper.
To serve, assemble patties in buns with the mushroom-onion topping, adding turkey bacon (if using) or desired condiments.