In a slikket, heat the olive oil and cook the mushrooms for 4-5 minutes until tender. Set aside.
In a bowl, mix together the mayonnaise, Dijon mustard, pepper, and garlic powder together. Oven the hoagie buns and spread the mayonnaise mixture over both sides of the buns.
On the bottom have of each bun 2 slices of ham, bacon and cheese. Then top each with 1/4 of the cooked mushrooms and place on the top of the bun.
Wrap each sandwich in aluminum foil. Bake at 375 degrees for 12 minutes. Remove from foil and serve. Serves 4.