Fry up bread in saute pan in a little butter. Don't forget it!
while bread is frying saute mushrooms and onions in separate pan in a little olive oil. wait till mushrooms get some color before you salt so they don't get soggy.
Mix all burger ingredients except feta together in a bowl using hands. Divide and shape into 2 patties. making a small dent in the center of each patty will help them keep their shape. By this point the bread should be nice and crispy; remove and set aside. Turn heat up to med./High, Fry burgers in same saute pan 5-6 min on first side. Flip and place feta on top; saute covered 5-6 min. May need to turn heat down a bit for second side.
Assemble burgers. I put a little dijon mustard and mayo on my bread, but it's optional. Place patty on one slice of bread, top patty with mushroom and onion saute top with second slice of bread. Serve with simple Romain and tomato salad.