Gently twist off the stems of the portobello mushroom caps.  Wash and dry the mushrooms and place them in a shallow baking dish. In a small bowl, mix together the avocado oil, lime juice chili powder, cumin, salt, and garlic.  Pour it over the portobello mushrooms and turn them over several times so they are well-coated.  Cover and allow to marinate 30 minutes or longer.

To make the chipotle mayo, put the mayonnaise, chipotle pepper, adobo sauce, and lime juice in a small blender or food processor and process until smooth.  Taste and add salt and pepper as needed.

Preheat your grill to high heat, about 400 degrees.  Oil the grates and then add the mushroom caps to the grill.  Allow the mushrooms cook about 5 minutes per side.  Baste with the remaining marinade liquid during the cooking time, and add the cheese during the last 1-2 minutes.

Assemble your burgers and top with the chipotle mayonnaise.  I added lettuce and tomato to my burger; pick your favorite toppings and enjoy!