In the bowl of your food processor fitted with the blade attachments, work in batches of 2 and pulse the cauliflower florets until coarsely crumbled. Then transfer the cauliflower to a rimmed, metal baking sheet.
Next add in all of the mushrooms and walnuts and pulse until coarsely crumbled. Then transfer to the baking sheet.
Lastly add in the small, roughly chopped onion and pulse until finely minced. Add the onion to the pan, drizzle with olive oil and season with homemade taco seasoning. Toss well until combined.
Divide the vegan taco mixture in half and transfer one half to the second rimmed, metal baking sheet.
Slide both pans onto the lower two racks of your preheated oven and bake for 10 minutes. Remove, stir, swap and rotate the pans on the oven racks. Continue baking for another 10 to 15 minutes.