Step 1

Stir orange juice, agave, soy sauce, olive oil, garlic and pepper together in small bowl. Place portobellos and onion strips in a shallow dish and pour marinade over top. Allow to marinate for at least 30 minutes at room temperature, or up to 12 hours in the refrigerator.

Step 2

Preheat oven to 400º. Place portobello mixture in baking dish, along with marinade. Bake 20 minutes, flipping once about half way through.

Step 3

Place all ingredients into food processor and blend to a uniform consistency, stopping to scrape down sides of bowl as needed.

Step 4

Divide portobello filling among shells and top with lettuce, tomato, avocado and chimichurri sauce.