Pull ground chuck out of fridge while sauteing mushrooms and onions
Add onions to pan and saute for 10 min
Reduce heat to lowest setting while grilling
Grill burgers for 5 minutes a side, flipping once. Press on the middle of the burger with you finger to get a feel for doneness. Burgers should be slightly firm in the center. Internal temperature should be around 135 F for a nice medium to medium rare burger.
Add cheese to burgers and place buns on the grill to toast them. Close the grill for 30 sec or so and you should be done!