How to cook salmon on spinach puree and mustard reduction

4.0 Salmon or monkfish, 1.0 Shallot or onions, 250.0ml Heavy cream, 2.0 Celery, Thyme, Bay leaves, 1.0Tbsp Mustard seeds, 150.0ml White wine, 0.0 Lemon, 150.0g Spinach, 0.25l Milk, 60.0g Butter, 1.0kg Potato, Nutmeg, Olive oil, Pepper, Salt