This is a game version of the classic mushroom Swiss burger you see in hamburger joints all over the country: A big ole' meat patty, topped with grilled or sauteed onions and mushrooms, served with cheese, usually Swiss. Mustard is the traditional accompaniment here.
Mix into the meat the dried mushroom powder and celery seed. Shape the meat into patties. When you do this, don't overwork your meat or your hamburgers will become tough and chewy. Crumbly is better than tough, to my mind. Also, press a little indentation into the center of each patty: This helps the patty keep its shape once cooked, because when meat cooks it tightens up and will turn into a ball shape if you don't have that indentation.
Get a large saute pan hot over high heat and add the mushrooms. Shake the pan frequently so they don't all stick, and cook the mushrooms until they release their water, about 2 to 4 minutes. As soon as the water has mostly bubbled away, add the olive oil, some salt and the sliced onion and saute until everything has nicely browned, about 6 to 8 minutes. Turn off the heat and set aside.
Salt your burgers and grill them (or cook them in any other way) to your liking. I like mine medium, so I grill over medium-high heat about 4 to 6 minutes per side. I only flip my burgers once. When you flip the burger, let it cook about halfway on the second side before slapping a slice of cheese on the patty. Close the grill lid to let it melt.
To build the burger, toast the buns if you want first. Paint with mustard or whatever, then lay down a patty. Top with the mushrooms and onions and have at it! Serve with a salad and a cold beer.