2. In a mixing bowl, combine olive oil and balsamic vinegar. Brush the mixture onto the portobello mushrooms.
3. Grill the mushrooms for about 5-7 minutes on each side until tender.
4. Place a slice of provolone cheese on each mushroom and let it melt.
5. Toast the ciabatta rolls on the grill pan until golden brown.
6. Assemble the sandwiches by placing the grilled mushrooms with cheese on the bottom half of the rolls. Top with arugula and tomato slices. Cover with the top half of the rolls.