Grilled Portabella and Bulgur Salad "Sandwiches"

With motor running, drop garlic into food processor and finely chop. Add tomatoes, parsley, oil, vinegar, rosemary, red-pepper flakes, 1/2 teaspoon salt, and 1/4 teaspoon pepper and pulse until tomatoes are finely chopped but not puréed.
Peel skin from portabellas using tip of a paring knife, pulling from edge of mushroom toward center. Gently scrape off gills from mushrooms with tip of a spoon. Toss portabellas with 1/2 cup vinaigrette in a 13- by 9-inch glass baking dish and let stand, turning occasionally, 20 minutes.
Prepare a gas grill for direct-heat cooking over medium heat; see Grilling Procedure .
Push 1 wooden pick into each onion wedge to secure for grilling. Toss onion wedges, bell peppers, and zucchini with oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Oil grill rack, then grill vegetables, covered, turning occasionally, until tender (6 to 8 minutes for bell peppers, 6 to 10 minutes for zucchini, 8 to 12 minutes for onion, and 12 to 15 minutes for portabellas).
Transfer portabellas to a plate and remaining vegetables to a bowl as cooked. Cool slightly, then remove wooden picks and coarsely chop onion, bell peppers, and zucchini.
While vegetables cook, pour boiling-hot water over bulgur with 1/2 teaspoon salt in a large bowl and let stand until water is absorbed and bulgur is tender, about 5 minutes. Drain in a sieve if any water remains.
Add vegetables to bulgur with pine nuts and remaining vinaigrette and toss.
Put one fourth of arugula on each of 4 plates. Top with upside-down portabella. Pack bulgur mixture into a cup measure, then unmold on top of portabella. Top with another portabella, right side up. Serve with an avocado quarter.