For this sandwich first I stir fry the spinach, onions, and garlic (use as much or as little as you want) over medium heat with about one teaspoon of spicy sesame oil, and about a tablespoon of olive oil. Use as much oil as you are used to using for stir fry. Stir fry until spinach is wilted and set aside.
Add portobello mushroom cap to pan and pour balsamic vinegar over mushroom while pressing it into the pan, flipping the mushroom every few minutes and making sure it soaks up the vinegar. Its better to use more vinegar here, if there is excess in the pan you can drain it out into the sink while cooking. Sometimes I will sprinkle some garlic salt on top while cooking. Continue this until the mushroom cap is pressed well. About 3-5 minutes.
Spread some mashed up avocado on toast and arrange ingredients accordingly.
Enjoy!