Coat Halibut in spices, salt & pepper. Set aside.
For the slaw, combine red cabbage, red onion, mango, jalapeño, cilantro and green onion; mix well and set aside.
Sor the sauce, combine avocado, tomatillo, lime juice, 2 T cilantro, 1/2 T salt and water in a blender. Blend until smooth, adding more water until desired consistency is reached.
Heat canola oil in a sauté pan over high heat. Right before the oil begins to smoke, add the seasoned fish. Saute until fish is cooked through and spices have become encrusted.
To serve, steam, then double stack the taco shells. Top with fish, slaw and tomatillo-avocado sauce.