• Whisk balsamic vinegar, garlic, soy sauce and oil together in a measuring cup.
• Place mushrooms in a gallon ziplock bag; pour sauce over and distribute evenly over mushrooms, expelling air before closing.
• Marinate in fridge 1 - 3 hours (or at room temperature for 1/2 hour), turning frequently.

To Saute

• Remove mushrooms from sauce; discard sauce. Slice mushrooms into 1-inch thick pieces.
• Spray a 12-inch non-stick sauté pan with olive oil cooking spray.
• Saute mushroom slices over medium-high heat, stirring often, until cooked through, about 10 - 15 minutes.
• Lay immediately over mixed greens, sprinkle with fresh ground black pepper and a little salt to taste. (Try one first to see if it needs salt.)
• Serve warm over mixed greens.

To Grill

• Grill caps on grill rack coated with cooking spray for about 10 minutes turning once halfway.
• Slice each mushroom into 4 pieces; serve warm over mixed greens.