In a large bowl, combine ground beef, salt, pepper and garlic powder; form mixture into 4 patties.
Preheat outdoor grill to HIGH. Place patties on grill then reduce grill temperature to MEDIUM-HIGH. Grill patties for 10 minutes per side or until they reach desired level of doneness.
Meanwhile, rub mushroom caps with olive oil; place them on the grill and cook for 5 minutes per side or until slightly tender.
In a large bowl, combine remaining ingredients (onion through honey); toss to combine. On each dinner plate, layer a mushroom cap, a burger patty and an even portion of beet salad.