Preheat the oven to 450F or 425F convection. Line two rimmed baking sheets with parchment paper. Mix together the spices (cumin, smoked paprika, coriander, garlic, and red pepper flakes, if using) in a small bowl.
Drain and rinse the chickpeas, then place them on a kitchen towel and dry them off as much as you can. Add cauliflower to one baking sheet and chickpeas to the other. Drizzle each sheet pan with 1 ½ tablespoons olive oil, half of spice mixture (about 1 ½ tablespoons), and ½ teaspoon salt. Toss each to coat, using your hands to really rub the spice mixture into the florets. Place the cauliflower on the middle rack and chickpeas on the lower rack. Bake for 20 minutes, or until the cauliflower is charred and chickpeas are crispy, switching the pans halfway through.
Meanwhile, line a plate with a piece of paper towel and make the walnuts. In a small or medium skillet over medium heat, add the olive oil and nuts. Once the nuts start to sizzle, reduce the heat to low and toast the nuts, stirring constantly, for about 3 minutes or until they start to turn golden (don’t burn them!). Remove from heat and use a spoon to transfer the nuts to your plate, leaving the oil behind. Sprinkle the garlic powder and salt over the warm nuts and toss to coat.
Warm up your tortillas in a pan until pliable, top with warm cauliflower and chickpeas, walnuts, yogurt, herbs and a squeeze of lemon.