How to cook cubed fish for a fish taco (tortilla)
Don't cut it up before cooking, do it after cooking. This is a common issue with many Mexican style recipes

Coat the fish with your spice mix, and cook as desired. When done, cut into cubes/chunks or flake onto your tortilla

Usually thinner fillets work best for this style in regards to surface area exposed to spices etc

Do the same for beef tortilla, spice and cook a steak, then slice it up thin and add it to your tortilla