grilled portobello mushroom burgers

Wow!  Hall of fame.  I never thought a mushroom burger could be so good.  But then again I'd never tried one before.  It was just about the best burger I've ever had.  Super gourmet and without the guilt.  Even better, my husband made dinner. :)

grilled portobello mushroom burgers (adapted from Skinnytaste)
2 portobello mushroom caps
2 tbsp balsamic vinegar
1 tablespoon soy sauce
1 tablespoon olive oil
1 tablespoon chopped rosemary
1 1/2 teaspoons steak seasoning
2 thick slices red onion
2 slices Provolone cheese
2 thin slices tomato
1/2 avocado, sliced thin
2 whole wheat buns

In a large bowl, whisk together vinegar, soy sauce, oil, rosemary and steak seasoning.
Peel off the tops of the Portobello mushrooms and scoop out the gills.
Place the mushroom caps in the bowl and toss with the sauce, using a spoon to evenly coat.  Let stand room temperature for 20-30 minutes, turning a few times.
Heat the grill or indoor grill pan over medium heat.  When hot, brush the grate with oil or lightly spray the grill pan.
Place the mushrooms on the grill, reserving marinade for basting.  Grill for 5 to 7 minutes on each side, or until tender, brushing with marinade frequently.
Top the mushrooms with cheese during the last minute of cooking.
While the mushrooms cook, grill the onions about 1 minute on each side and grill the buns until toasted.
To finish, place the spinach and grilled mushrooms on the buns and top with the grilled onions, sliced tomato and avocado.