Pat fish dry, then coat both sides with flour, reserving the excess flour. Melt the butter in a pan big enough to hold the floured fish, then cook over a moderate heat 3 to 4 minutes per side until lightly browned.
Lift fish out of the pan and add reserved flour to the remaining butter. Add the orange juice, and bring to the boil, then add water and stock, and simmer until thickened. Taste and adj