1. In a large nonreactive skillet, warm the oil over moderately high heat. Add the fish and cook until whitened but still opaque in the center, about 2 minutes per side. Add the onion and garlic, reduce the heat to moderate and cook, stirring, until the onion is wilted and the fish breaks into large flakes. Season with salt and pepper and set aside.
2. In a medium skillet, melt 1/2 teaspoon of the butter over moderately high heat. Add 1 tortilla and flip it to coat with butter. Cook the tortilla until blistered and lightly toasted, 1 minute per side. Repeat with the remaining butter and tortillas; wrap the tortillas in foil to keep warm.
3. Gently reheat the fish mixture and transfer it to a serving dish. Put all the accompaniments in small bowls and let guests assemble their own tacos.
Serve With: Yellow rice with black beans and Mexican beer with lime wedges.