Preheat the oven to 450˚F.
Separate the cauliflower into florets and spread out on a parchment-lined sheet.
Drizzle with oil and toss with your hands to coat. Sprinkle with salt and roast for 20 minutes, until soft and golden.
Meanwhile, rub the cod with chili powder and salt.
Place on the baking sheet with the cauliflower and return to the oven for 10 more minutes, until the edge flakes with a fork but the middle is still moist.
In a blender, combine the yogurt, mayonnaise, cilantro, jalapeño, garlic and lime juice with a big pinch of salt and blend until smooth.
Serve the flaked fish, roasted cauliflower and sliced Brussels sprouts in warmed tortillas, drizzled with the cilantro sauce.