Slice the mushrooms into bite-sized pieces.

Heat the oil in a pan over medium heat. Add the mushrooms, onion powder, cumin, and salt/pepper (to taste). Stir until well coated. Cook for 3 minutes, stir the pan, and cook for another 3 minutes, until browned and soft.

Take a leaf of lettuce, place the mushrooms in it and top with guacamole and salsa. Serve.

Note: to store, keep the mushrooms in an airtight container. When ready to eat, re-heat the mushrooms and build the taco.