In a mixing bowl, combine the finely chopped Lion's Mane mushroom, red bell pepper, green onion, mayonnaise, Dijon mustard, Old Bay seasoning, breadcrumbs, beaten egg, salt, and pepper.
Mix the ingredients until well combined. The mixture should hold together when pressed.
Shape the mixture into small patties, about the size of crab cakes.
Roll each patty in breadcrumbs, pressing gently to adhere.
Heat olive oil in a skillet over medium heat.
Carefully place the mushroom patties in the skillet and cook for about 3-4 minutes on each side, or until golden brown and crispy.
Remove from the skillet and place on paper towels to absorb excess oil.
Serve the Lion's Mane "crab" cakes with a side of tartar sauce or a squeeze of lemon.