Enjoy these for your next Taco Tuesday.
Cut onion into a small dice. Mince mushrooms by hand, or in a food processor. Mince walnuts by hand, or in a food processor. (Do separately from mushrooms).
In a large skillet, heat oil over medium-high heat. Add all but 3 Tablespoons of diced onion and cook, stirring frequently, until starting to brown, about 5 minutes. Add minced mushrooms, tomato paste, spices, and salt, and cook for about 5 minutes.
Once the liquid has mostly evaporated, add minced walnuts and cook for about 2 minutes.
Chop tomato, avocado, and lettuce for topping. Prepare cilantro-lime gremolata, and cut limes into wedges. Use reserved 3 Tablespoons diced onion for topping.