Heat skillet to medium-high.
Rub fish with oil, and coat with blackening spice.
Sear fish top-side down in skillet for 4 minutes until blackened. Flip fish and cook for an additional 3 minutes. Let cool for 5 minutes before slicing.
Heat oven to 500° F.
Add olive oil to plate, and season with salt, pepper and garlic. Dip bread in oil mixture, and toast in oven until nicely browned.
In a separate bowl, combine lime juice, water, cilantro and sour cream to make Cilantro Sour Cream.
To assemble, place avocado slices on toast, and drizzle with Cilantro Sour Cream. Top with sliced catfish, tomatoes and onion.