Place the water and the cornstarch in a small saucepan and whisk until combined. Place the pot on high heat and just as it begins to boil, reduce the heat to a simmer and add the mustard. Season with sal and pepper. Cut the butter into small pieces and whisk it in slowly, continuing to whisk until you obtain a smooth sauce, about ten minutes. Do not allow sauce to boil! Place the sauce in a pre-heated wide mouth vacuum bottle while you prepare the fish. Place the fish fillets in a non-stick saute pan. Pour in the fish stock and bring to a boil. Lower the heat to a simmer and poach about 8 minutes. Lift the fish out carefully with a spider or large slotted spoon, dry with a paper towel if necessary and place in the center of a pre-heated dinner plate. Ladle the mustard-butter sauce over and around it, and garnish with chopped parsley and lemon slices.