Cut the loaf of bread in half and carefully remove the excess bread from both sides of the loaf, making sure not to break through the crusty outside.
Set aside.
Make the marinade by whisking the ingredients together.
If you plan to use large portobello mushrooms, marinate all the vegetables in a large ziplock bag. If you use smaller mushrooms and intend to saute them separately, as I did, divide them and place them in a separate ziplock with a quarter of the marinade.
Allow the vegetables at least an hour to marinate.
If sauteeing the mushrooms in a skillet, do so over medium-low heat until browned.
If grilling the mushrooms with the other vegetables, preheat the grill to 400 degrees, place the mushrooms, zucchini, summer squash, onion, and red bell pepper on the grill, and cook for 5 minutes on each side.
Then, place the bread on a baking sheet and broil for 3 minutes on low. Watch the bread, as it may cook quickly (oven broilers differ in strength).
Then, begin stacking the vegetables, starting with the mushrooms, then eh zucchini, summer squash, onion, and red pepper.
On the opposite side of the bread, smear the vegan southwest dressing, and layer tomato, spinach, and avocado. Next, fold the sandwich, cut it into serving sizes, and serve it with a pickle and a salad. I served the portobello mushroom sandwich with German potato salad.