Preheat the oven to 350 degrees. Line a baking sheet with aluminum foil or parchment paper. Brush some oil over the foil.
Line all the slices of mushroom flat on the foil in a single layer. Season on top with some salt and freshly ground black pepper.
Bake the mushrooms for about 15-20 minutes. Remove from the oven, and carefully flip each slice of mushroom. Use a small spoon to help flip if needed.
Bake the mushrooms again for an additional ~15-20 minutes to crispy both sides. In the last 10 minutes of baking, be sure to check to make sure they do not burn. If they seem crispy enough before the 20 minutes are done, remove them from the oven.
Place all the crispy mushrooms in a bowl lined with a paper towel. When cool enough to handle, toss with a little garlic powder. Feel free to add a little more salt and black pepper to taste if needed. Set aside (refrigerate if using at a later time).
To Prepare the Sandwich:
In a non-stick pan on medium heat, grill two slices of bread on both sides, so that they become crispy, using some vegan butter.
On one slice of grilled bread, spread a layer of the prepared Chipotle green onion aioli. Place a layer of crispy mushrooms on top, romaine lettuce, 4 slices of tomato, and 4 slices avocado. Spread another layer of the prepared Chipotle green onion aioli on the other slice of grilled bread and sandwich on top.
Repeat to make three more sandwiches. Cut the sandwiches in half and enjoy!
Notes
Make-Ahead Tip - Prepare the crispy mushrooms and chipotle aioli ahead of time.