- Add mushrooms to a food processor and pulse a few times to break them up. Add walnuts, soy sauce, lime juice, cumin, smoked paprika, oregano, garlic powder and cayenne. Pulse until mixture is shredded, but walnuts are still visible.
- Transfer mushroom walnut mixture to a lightly greased skillet over medium heat. Cook for 5-7 minutes to heat through, stirring occasionally.
- Spread a couple tablespoons of mashed avocado on tostadas. Top with mushroom taco meat, tomato, red onion, cilantro and serve with fresh lime.