Scrape out and discard the gills of each mushroom. Cut caps into 1/2-inch thick slices. In a small bowl, mix together the oil, vinegar, oregano and fennel, reserving 2 tablespoons. Place the sliced portobellos in the marinade for 20 minutes. Mix together the onion and remaining marinade in a small bowl and set aside.
Place tomatoes, garlic, parsley, basil, pine nuts, olive oil and Parmesan in a food processor and blend until the mixture is a spreadable paste, adding water as needed. Season to taste with pepper and set aside.
Heat a large skillet over medium-high heat. Remove the mushrooms from the marinade and sauté for 5-10 minutes until just tender. Remove from heat. Season with salt and pepper.
To assemble sandwiches, place provolone slices on the bottom half of each ciabatta roll, then divide the sun-dried tomato spread, sautéed mushrooms, marinated onions and lettuce evenly among the four sandwiches. Top with the other half of the roll and serve.
Lightly grill the ciabatta rolls before assembling the sandwiches and serve with a tangy bean or balsamic potato salad. Olive lovers will appreciate a smear of tapenade in their sandwich. In a hurry? Use a prepared pesto.
522 calories, 23 g. fat, 24 mg. cholesterol, 797 mg. sodium, 60 g. carbohydrate, 9 g. fiber, 23 g. protein