Prepare marinade by mixing 1/4 cup olive oil, balsamic vinegar, and Italian seasoning together. Slice portobello mushroom into longer easy to eat pieces. Place in marinade and make sure all pieces of the mushroom get covered in marinade. Set aside.
Heat skillet on med-high and add 1 Tablespoon olive oil, black pepper, and garlic clove. When garlic becomes tender add onion and salt. Heat until onions are transparent. Add spinach and cook until somewhat wilted. Add mushrooms and leftover marinade to skillet and mix around. I also had just a touch more olive oil. Let cook for approx. 3 minutes then stir mixture around again. Let cook for approx. another 3 minutes. Meanwhile, toast bread in toaster or oven. When done toasting, place bread on skillet face down. Press down with fork or spatula for about 30 seconds so the flavor and seasonings in skillet get on the bread. Put bread on plate with seasoned side face up. Spread Vegenaise on bread. Take mixture from skillet and put on one side of the bread. Spread veggie shreds on top. Close sandwich and enjoy!
I eyeballed measurements for this recipe when I made it so feel free to add (or not add) what you feel would taste good.