Heat the leeks or scallions and wine in a saucepan, until the liquid is reduced to about 3 tablespoons. Stir in the veloute. Cook over low heat for 5 minutes. Remove from heat. Blend the lemon juice and soft butter together and stir in the sauce. Add seasoning, if needed. Makes about 2 3/4 cups of sauce. Serve on fish steaks, baked fillets, or broiled salmon.