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 8-10 minutes. The point is to slightly undercook the lentils: they should be mostly cooked but not fully cooked with a distinct bite, as they’ll get cooked later and the taco meat will get mushy if fully cooked. Drain the lentils thoroughly.
Transfer the cooked lentils to a food processor and blend a few times, followed by a few pulses, until the lentils are partially blended with a few pieces still intact. Transfer the lentils to a large bowl and set aside. Wipe out the food processor bowl.NOTE: Watch the Youtube video at the 03:07 mark to see the desired texture of the blended lentils.
While the lentils are cooking, toast the walnuts. Add to a dry skillet over medium heat and toast for 5 minutes until they smell toasty and are darker in color.
Back in the food processor, add the toasted walnuts. Blend briefly until you have fine crumbs, but don’t over-process, or it will start to turn into walnut butter. Add chopped mushrooms and onions to the food processor and blend a few times, followed by a few pulses until they are very finely chopped. NOTE: Watch the video at the 04:00 mark to see the desired texture.
Make the taco meat filling. Add the walnut-vegetable mixture to the bowl with the blended lentils. Then add the flaxseed meal, nutritional yeast, salt, pepper, smoked paprika, taco seasoning, porcini mushroom powder (if using) and beef broth flavoring (if using). Use your hands to mix the taco meat until the mixture comes together with your hands. It should easily come together but not be very sticky. If the mixture is too sticky, add a few tablespoons of all-purpose or gluten-free flour and toss to combine.
Cook the taco meat. Heat a large (12-inch) nonstick skillet over medium/medium-high heat with a tablespoon of olive oil. Once hot, add about 1/3 to 1/2 of the mixture to the pan (do not overcrowd) and break the taco meat up with a wooden spoon or angled spatula into as small pieces as you can. Cook for 4-5 minutes, or until browned and slightly crispy. Repeat with remaining taco meat.