Makes 2 burgers:

2 large Portobello mushroom caps, stemmed
3 T balsamic vinegar
1 T olive oil
1 t minced fresh garlic
½ t pepper
¼ t salt
¼ c crumbled feta, gorgonzola or blue cheese
2 T light mayo
1 t balsamic vinegar
Baby spinach leaves
Sliced tomato
2 burger buns
Plenty of napkins!

Scrape gills from mushroom caps.  Combine 3T vinegar, oil, garlic, salt and pepper in a zip-top freezer bag.  Add mushrooms, turning to coat. Chill 30 minutes to 1 hour, turning occasionally.  Remove mushrooms from marinade and discard marinade.  

Grill mushrooms over med-high heat until tender (3-4 minutes each side)

Remove from grill and immediately sprinkle feta cheese to underside of each cap. 

Stir together the 2T mayo and 1t vinegar; spread on burger buns.  Build burger with the spinach leaves, tomato and mushroom and serve immediately.