Step 1Combine the oil, garlic, tamari and black pepper to taste in a large shallow bowl. Add the portobello caps, turning to coat both sides. Let stand 30 to 60 minutes.
Step 2Roast the bell pepper under the broiler or over a flame until it is evenly charred on all sides. Cool, then remove the skin, seeds and tough ribs. Cut into thin strips and set aside.
Step 3Lay the portobello caps stem side up on broiler pan and drizzle with any remaining marinade. Broil 5 minutes, then turn over with tongs and broil 5 minutes longer. Turn once more, dredging the mushrooms through the pan juices, and broil 2 minutes longer.
Step 4While the portobellos are cooking, slice the ciabatta rolls in half and lay the halves out on the work surface. Spread the bottom half of each with pesto.
Step 5Place a hot portobello onto each pesto-covered ciabatta. Divide the cheese among the four sandwiches. Place an equal amount of pepper slices over each, then top with arugula. Press the top halves of the rolls onto the sandwiches. Slice in half diagonally and serve at once.