Soak top and bottom of 3 1/4 quart clay cooker in water about 15 minutes; drain. Line bottom with parchment paper.
Wipe salmon steaks with damp toweling. Coat with mixture of flour, salt, curry and paprika. Place in cooker. Arrange the potatoes, standing on edge around the salmon. Mix butter, lemon juice and mustard seeds and drizzle over salmon and potatoes.
Place covered cooker in cold oven. Set temperature to 450°F Bake until salmon is firm and potatoes are tender, about 35 minutes. Remove cover and bake until salmon is brown, about 10 minutes.
Drizzle salmon and potatoes with cooking liquid; sprinkle with green onions.