Heat the olive oil in a large skillet or frying pan over medium heat. Add the finely chopped mushrooms and cook, stirring occasionally, until they release their moisture and begin to brown, about 8-10 minutes. This process helps develop their savory flavor.

Add the finely chopped onion to the skillet with the mushrooms. Cook until the onion is softened and translucent, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.

Stir in the finely chopped walnuts, chili powder, cumin, smoked paprika, oregano, salt, and black pepper. Cook for 1-2 minutes, stirring constantly, until the spices are fragrant and evenly distributed throughout the mushroom and walnut mixture.

Pour in the vegetable broth or water. Bring the mixture to a simmer, then reduce the heat to low, cover the skillet, and let it cook for about 5-7 minutes, or until most of the liquid has evaporated and the mixture has a thick, "meaty" consistency.

Remove the skillet from the heat. Stir in the fresh lime juice for a touch of brightness. Taste and adjust seasoning with salt and pepper if needed.

Warm the taco shells or tortillas according to package directions. Spoon the mushroom and walnut "meat" filling generously into the prepared taco shells or tortillas. Serve immediately with your favorite taco toppings.