In a bowl stir together mustard powder and water to make a paste.
In a saucepan combine vinegar, wine, onion, shallots, garlic, bay leaf, peppercorns, and juniper berries and bring mixture to a simmer over moderate heat. Simmer mixture until reduced by two thirds. Strain mixture, cover and chill.
Stir the chilled vinegar reduction into the mustard paste.
Add the lemon juice, salt and sugar and stir to combine. Let mixture stand for at least 20 minutes.
Transfer the mustard mixture to a saucepan, bring to a simmer over low heat and cook for 15 minutes. Remove from heat and allow to cool.
Transfer to a sterile jar and seal tightly, and store on a dark, cool shelf for at least a month or up to 6 weeks, before using. Mustard will mellow with age.
Mustard should be refrigerated once open and will keep for 6 months.
Yield: about 2 cups