Preheat oven to 350 degrees F and add walnuts to a baking sheet (can be unlined). Roast walnuts until slightly darkened in color and fragrant, approximately 10 minutes (start checking at 8).
Remove walnuts and place in a heatproof bowl and set aside. Turn oven up to 400 degrees. Now line baking sheet with parchment paper. Add cauliflower and mushrooms to the baking sheet. Drizzle oil over vegetables and toss gently to coat.
Roast veggies for 20 minutes, until the cauliflower florets have golden brown edges. While roasting, make Taco Spice Mixture. Add 1/4 cup of spice mixture, walnuts and nutritional yeast to a food processor. Pulse just 1-2x to incorporate.
When veggies have finished roasting, allow to cool until they are easy to handle with your fingers but still warm to touch. Add to food processor and pulse another few times, until mixture is incorporated but still has a chunky texture. Taste and stir in extra spice mixture and salt as you like—I personally use all the spice mixture and add an extra 1/2 teaspoon salt.
Serve immediately and enjoy! Leftover “meat” will keep tightly sealed in the refrigerator up to 5 days or frozen for 1 month.