This French dish usually is made with salt cod, but sablefish (also sold as black cod) is a much more environmentally sustainable choice. You often can find smoked sablefish in Jewish delis, and many upscale grocery stores carry it as well. Because the sablefish is smoked and then cooked again, its final texture ends up resembling that of the reconstituted salt cod.
Combine the sablefish, potato, and garlic in a small saucepan and barely cover with water. Bring to a simmer and cook until the potato pieces are soft and beginning to fall apart, about 12 minutes. Drain, reserving about 1/2 cup of the cooking liquid.
Preheat broiler if using to toast the bread.
Transfer the potato, fish, and garlic to a medium bowl and begin to mash with a whisk. Add the olive oil in a slow stream, whisking constantly to incorporate it as you add it, then whisk in about 3 tablespoons of the cooking liquid until the purée has the consistency of peanut butter. (The brandade will keep for several days, tightly covered, in the refrigerator.)
Brush the bread slices with a little oil and toast under the broiler or in a toaster oven until golden brown. Serve the brandade in a bowl, topped with a drizzle of olive oil and a sprinkling of herbs, if desired. Offer the toasted bread alongside.
Serves 4 as a canapé or light appetizer