Portobello Mushroom Sandwich

1. Combine the mayonnaise, horseradish, and sprinkle a little salt. Mixed well and set aside. Chop the garlic cloves and add them to the olive oil. Once combined, separate mixture into 2 bowls.
2. Take one of the bowls containing the olive oil & garlic mixture and add the balsamic vinegar, and salt &  pepper to taste. Set aside.
3. Clean the portobello mushrooms with a damp towel as needed before slicing them into ½ inch pieces.
4. In a large bowl, combine the sliced mushrooms & the olive oil balsamic vinegar mixture. Gently mix the mushrooms until they are fully coated with the marinade. Set aside.
5. Slice the ciabatta rolls, brush with remaining olive oil & garlic mixture and toast in oven until golden brown.
6. Place the mushrooms in a microwave-safe dish and microwave loosely covered for 2-3 minutes, until the mushrooms are fully cooked.

Sandwich assembly:

1. Spread the mayonnaise mixture onto the ciabatta rolls evenly & layer the sandwich ingredients in the following  order.
2. Layer the mushroom pieces on the ciabatta roll bottom.
3. Add a slice of provolone cheese on top of the mushrooms.
4. Add a large piece of roasted red pepper.
5. Add ½ cup of spinach leaves.
6. Top with red onion pieces.
7. Place the ciabatta roll top, slice the sandwich in half if desired before serving.