Preheat oven to 475°F. Coat cauliflower with 1 tablespoon olive oil and dust with salt and cayenne pepper; place on a baking sheet and roast for 25–30 minutes or until lightly browned.
Reduce oven temperature to 350°F and toast walnuts for 5 minutes or until lightly browned. Add to pan with the cauliflower.
Heat remaining olive oil in a large skillet over medium-high heat. Add onion and sauté for 5 minutes or until lightly browned. Add garlic and jalapeño cook for 2 minutes more. Stir in water, tomato paste, chili powder, cumin, oregano, and salt; cook until mixture is thick and excess water has cooked off. Stir in cauliflower mixture.
Spoon mixture into warmed tortillas and top with cabbage, cheese, walnuts, and cilantro. Serve with lime wedges.
NOTE: Cauliflower mixture may be ground in a food processor so that it resembles the texture of ground meat.