In pan, melt 2 tablespoons of the butter and saute onion until soft. Add the paprika and cayenne and cook gently for 2 minutes longer. Add salt, lemon juice and remaining butter and heat until melted. Set fish on a broiler rack and brush with the paprika mixture. Broil about 5 cm from heat for 4 to 5 minutes, brushing with more of the paprika mixture. Turn; brush with remaining butter and broil 4 to 5 minutes longer. Fish should be browned and flake easily.