Combine the ground beef, salt, pepper, garlic powder, and balsamic vinegar in a bowl and mix until well combined.
Divide into two equal portions and shape into patties.
Remove the gills from the portobello mushroom caps.
Preheat BBQ grill or grill pan to medium-high heat. Spray grates with non-stick cooking spray.
Cook the burgers for about 5 minutes on each side (for medium-well) and the portobello mushroom caps for about 4 minutes on each side, until slightly softened.