Heat the chicken stock to boiling in a saucepan, then reduce the heat and hold at a simmer.
Heat the half the butter in a large sauté pan or skillet over medium heat. Sprinkle the fish with salt and pepper, add it to the pan, and sauté it quickly. If you are using crab-meat, just warm it in a little butter for a minute. If you don’t have a very large pan, cook it in batches.
Heat the remaining butter and the oil in a wide heavy saucepan over low heat. Add the onions and cook until translucent, about 5 minutes. Add the rice and cook, stirring to coat with butter, until opaque, about 3 minutes.