In a small bowl, stir together the spices.
Heat enough oil to generously coat a cast iron skillet over medium-high heat, about 3 to 4 tablespoons. Coat each side of each fillet generously with the spice mixture to encase the fish. Pat the spices gently into the flesh. You may not need to use all of the spice mixture.
When the skillet is completely hot and the oil begins to smoke, carefully place one or two of the tilapia fillets on the skillet.
Allow the fillet(s) to cook until the sides have firmed up and the bottom is blackened, about 4 to 5 minutes depending on the thickness of the fillet.
Carefully flip fish to the other side and cook an additional 4 to 5 minutes, until both sides are blackened.
Repeat for remaining fillets, adding oil to the skillet first if necessary.
Serve fish with desired amount of fennel slaw, avocado slices, and brown rice.
