Sprinkle fish with 1/4 tsp chili powder and 1/4 tsp salt.
Make the chili-lime cream: in a bowl, squeeze 2 tsp of lime juice from one half of lime or one small lime. (add more if needed). Mix with sour cream, garlic powder and remaining chili powder. Set aside.
Cut remaining lime into wedges for serving.
Place fish on unheated greased broiler pan. Tuck under thin parts.
Place shells on baking sheet on bottom rack of oven.
Broil fish 4" from broiler 4-6 minutes per half-inch thickness until it flakes with a fork.
Break fish into chunks. Combine fish, slaw mix and cream.
Spoon onto shells. Serve with avocado and tomatoes.