PREHEAT oven to 350 degrees and line a baking sheet with parchment paper. Remove gills and stems from cap to make space for filling using the tip of a spoon (do carefully if you want to keep the mushrooms looking nice). Place onto parchment paper open side facing down. Bake 10 minutes. Remove from oven and set onto a plate.
WHILE mushrooms bake, heat olive oil in a large pan oven medium-high heat. Add bell pepper and onion, cook 5 minutes stirring once. Add sliced steak, cumin, oregano, garlic powder and 1/4 teaspoon sea salt. Cook 5-7 minutes longer depending on desired doneness of steak.
MASH avocado using a fork, stir in cilantro and remaining sea salt. Fill portobello caps with steak and veggie mixture (about 1 cup filling per cap). Top with a dollop (1 tbsp) of guacamole on each cap before serving.