Cook the lentils. Bring some water to a boil in a saucepan and lightly salt the water. Once boiling, add the lentils. Reduce the heat to a simmer and cook the lentils for just 8 to 10 minutes. You want to slightly undercook them: they should be mostly but not fully cooked with a distinct bite, as they’ll get cooked later.
While the lentils cook, toast the walnuts. Heat a dry skillet over medium-low heat. Add the walnuts and toast for 4 to 5 minutes until they smell toasty and are darker in spots.
Drain the lentils well, then add to a food processor. Blend for several seconds, followed by a few pulses, until partially blended with a few pieces still intact. Transfer to a large bowl and set aside. Wipe out the food processor bowl.TIP: Watch this video at the 11:26 mark to see the desired texture. Add the toasted walnuts to the food processor. Blend briefly until you have fine crumbs, but don’t over-blend into walnut butter. Add the roughly chopped mushrooms and shallots to the food processor with the walnuts. PULSE several times until mushrooms and shallots are very finely chopped (blending continuously can make the mixture too wet). NOTE: If you have a small food processor, do this in 2 batches. Overfilling the food processor may result in a mushy filling. TIP: Watch this video at the 11:39 and 11:46 to see the desired texture. Make the taco meat filling. Add the walnut-veg mixture to the bowl with the lentils. Add in the flaxseed meal, nutritional yeast, 1/2 tsp kosher salt, pepper, smoked paprika, 2 tablespoons taco seasoning, plus mushroom powder and beef broth seasoning (if using). Use your hands to mix until the mixture comes together with your hands. It should easily come together but not be very sticky. If the mixture is very sticky, add a tablespoon or two of flour and toss to combine.Taste, adding salt as needed (I usually add a bit more salt). Cook the taco meat in batches. Heat a large (12-inch) nonstick skillet over medium/medium-high heat with 2 tablespoons of olive oil. Once hot, add ⅓ to ½ of the mixture to the pan (to avoid overcrowding and mushy texture) and break the taco meat up with a spatula into as small pieces as you can. Cook for 4 to 6 minutes, stirring occasionally, or until browned and slightly crispy. Repeat with remaining taco meat, adding more oil as needed. Warm the tortillas. Add a few tortillas to a cast iron (or other) skillet over medium-high heat. Warm for about 30 seconds per side. Stack and wrap tortillas in a dish towel to keep them warm.
Assemble. Dollop some Avocado Crema onto each warmed tortilla and spread out. Top with taco meat and Spicy Cabbage Slaw. If desired, sprinkle a bit of cilantro onto each taco, and serve with lime wedges.