Preheat the oven to 350 degrees F. Place the bulgur in a heatproof bowl. In a sauce pan, bring the chicken broth and 1 1/2 cups water to a boil. When the liquid has come to a boil, pour over the bulgur. Let sit until the bulgur absorbs the liquid, about 20 minutes.
Meanwhile, prep the vegetables: Cut off the top third of the bell peppers. Remove the seeds and ribs leaving a 'cup' for the stuffing. Dice the flesh from the top 1/3 and reserve. Cut a 1/3 of the zucchini lengthwise to reveal the seeds. Scoop out the seeds to create a boat. Dice the remaining 1/3 pieces. Slice the tomatoes in half and scoop out the seeds to create a cup.
In a large saute pan, heat the oil over medium heat. Add the chopped onion and sweat until translucent. Add the reserved chopped peppers, squash, mushrooms, and garlic to the pan. Season the vegetables with salt and pepper. Cook until the vegetables are tender, about 10 minutes. Drain the cooked vegetables, if necessary, and add to the cooked bulgur.
Sprinkle salt and pepper on the inside of the vegetable 'cups.' Stuff the vegetables with the bulgur mixture, place in a shallow baking dish, cover with foil, and bake for 45 minutes. Uncover and sprinkle with grated Parmesan cheese, then bake for an additional 10 minutes, or until the cheese is brown and melted.
You may need to drain the excess liquid from the bowl of bulgur, too. Stir in the chopped tarragon.