Add oil to a medium pan and put on a medium-high heat until hot
Add mushrooms and onions if using. Cook for 6 - 8 minutes until onions and mushrooms are softening.
Turn heat to high and add sauce to mushrooms and spices to pan. Cook for about another 5 minutes, until the mushrooms give up their liquid and the onions are just about browning. We are doing this so the pan gets really hot to char the bell peppers next while getting the sugars from the balsamic or sauce to caramelize.
Remove mushrooms and onions and put aside.
Add the green peppers to the now hot pan and cook fast for 5 to 7 minutes, until skin is blistering or charring.
Mash the avocados a little before adding all ingredients except lime juice and salt.
Add lime and salt periodically while mashing, tasting as you go to adjust the seasoning.