Preheat charcoal or gas grill to medium-high heat (about 350-375F).
On a flat surface, brush Portobello mushroom caps with olive oil and sprinkle with salt on both sides.
Grill, for 3-4 minutes per side. Remove from grill and assemble the burger, spreading Avocado ranch on both the bottom and top buns, layering lettuce, grilled Portobello, avocado slices, black bean salsa, and additional avocado ranch dressing if desired.
Notes
You may substitute store-bought black bean salsa or your favorite salsa for the black bean and corn salsa.