How to Make It
In a skillet, toast the coriander and cumin seeds over moderate heat until fragrant, 1 minute; transfer to a spice grinder and let cool. Finely grind. In a bowl, mix the coriander, cumin, thyme, oregano, chile powders, salt, sugar, mace, cinnamon and nutmeg.
In a food processor, puree the onion, garlic and poppy seeds. In a casserole, heat 3 tablespoons of the oil. Add the onion mixture and cook over moderately high heat, stirring, until the moisture has evaporated and the onion puree begins to brown, 5 minutes. Add the remaining 1 tablespoon of oil and the spice mixture and cook until fragrant.
Arrange the catfish in the casserole in a single layer. Pour in the water, cover and cook over low heat, stirring occasionally, for 5 minutes. Transfer the fish to a bowl. Boil the sauce, uncovered, until it has thickened, 5 minutes longer. Return the fish to the casserole and keep warm.
In a large bowl, toss the onion with the lime juice and season with salt. Add the diced avocados and toss.
Scoop the rice into bowls. Spoon the fish on top and serve with the avocado salad.