Melt 1-1/2 cups of the butter in a large saute pan over medium heat; add curry powder and cook 2 minutes. Stir in wine; gently lay fillets in pan. Simmer until cooked halfway through. Remove fillets from pan; cool, cover and refrigerate until ready to finish cooking.

 

Make beurre manie with remaining 1/2 cup butter and flour. Transfer curry poaching liquid to saucepan over medium heat; stir in milk, cream, lemon juice, and beurre manie. Cook stirring until thickened. Season to taste with salt and pepper. Keep warm until ready to serve.

 

For each serving: Place one fillet in gratin dish; top with 6 Tbsp mushrooms and 2 Tbsp onions. Top with generous 1/4 cup curry sauce. Cover and bake at 450 degrees F for approximately 8 minutes until fillet is opaque throughout and flakes easily and sauce is hot. Garnish with curry condiments. Accompany with 1 cup rice.