Dad's Mushroom Masterpiece

• 475 ml water are needed to steam the fillets
• 240 gr trout steaks
• 170 gr mushrooms (fresh)
• 80 ml milk
• 60 ml sour cream
• 30 gr onion (chopped)
• 28 gr butter
• 16 ml dijon mustard
• 16 ml wine (white)
• 8 gr all purpose flour