|¾ cup||Balsamic vinegar|
|2 Tablespoons||Olive oil|
|2 Tablespoons||Mendocino Mustard Hot & Sweet|
|4 large||Portobello mushroom caps, approximately 4 ounces each|
|8 slices||Bacon, optional|
|1 cup||Sliced onion|
|2 Tablespoons||Mendocino Mustard Seeds & Suds|
|4 large||Hamburger buns|
|4 large||Tomato slices, 1/4" thick|
|4 slices||Smoked cheddar or swiss cheese, about 1/8" thick|
Combine first 3 ingredients in a large zip-top resealable plastic food bag. Remove excess stems from mushroom caps and add to bag, turning gently to coat with marinade. Seal and refrigerate 12 to 24 hours.
Spray broiler or grill with non-stick spray and preheat.
Cook bacon in a large skillet over meduim/high heat until crisp. Remove from pan. Drain skillet, but do not wipe clean. Add sliced onion to skillet. Cook until golden brown, stirring frequently (10 - 15 minutes).
Remove mushrooms from marinade; discard marinade. Place mushrooms on grill grid or broiler pan about 4" from medium/hot heat. Broil or grill 5 - 6 minutes, turning once.
In a small bowl, stir together mayonnaise and Mendocino Mustard Seeds & Suds. Spread equal amounts on the inside of top halves of buns.
Top each bun bottom with 1 lettuce leaf, a tomato slice, mushroom cap, cheese slice, 2 slices of bacon and one-quarter of the cooked onion. Top with lettuce leaf and cover with top bun.