Marinate the portobello in the vinegar and olive oil mixture for 1 hour.
Slice into 5 to 6 pieces.
In a sauté pan over medium heat, sauté mushrooms in butter with onions, garlic and red bell peppers for 1 to 2 minutes until onions start to caramelize.
Place cheese into the sliced baguette. Bake 1 minute in a 350 degree F oven until cheese is melted.
Deglaze sauté pan with Balsamic vinegar and toss in spinach. Place all ingredients inside baguette. Top with tomatoes.Makes 1 Source: