Wipe mushroom caps clean and remove stem.
Combine balsamic vinegar, oil, worchestershire sauce and garlic. Cut mushroom caps into 3/4" slices, and add to marinade. Let sit in marinade for 15-20 minutes
Heat grill or grill pan to medium-high heat. Spray with nonstick spray to prevent it from sticking Add jalapenos, onions, and burger buns flat side down. Turn jalapenos ever 3-4 minutes, so all sides get charred. Turn onions after 6-7 minutes. Check buns, and remove when toasted to your desired level
When jalapenos are done, remove from heat and allow to cool for a couple of minutes. Cut off end, and seed jalapenos. For more heat, you can leave seeds in. Cut into long strips so they fit on the burger
Peel and seed avocado. Cut into large dice, and mash. It's up to you how mashed you want. I do not add any seasonings
Lay buns toasted side up on platter, and add mayo. Add baby spinach, tomato, portobellos, avocado mash, and jalapeno slices. Finish burgers with top buns