In a large skillet, heat the olive oil over medium-high heat. Add the cauliflower and the onion and sauté, stirring often, until the cauliflower browns in spots, about 10-15 minutes. Add the cumin, coriander, chili powder, garlic and walnuts and mix well. Cook for another 2 minutes while stirring. Taste and season with sea salt and pepper. Stuff in tortilla shells, garnish with your favorite taco toppings or with Avocado Cream, Pickled Red Cabbage and Cashew Sour Cream. Bon appétit!