Original Recipe
[Sauce for fish]
Hierophile, 13th century
For fresh fish, one should use, as it has been indicated for the month of June, all those that have delicate flesh, and these may be seasoned with a mustard sauce… [Master Demetrios, Byzantine Cuisine]

Ingredients:

• 3 Tbsp. Ground mustard seed
• 2/3 c. white wine vinegar
• 2 large garlic cloves, minced
• 3/4 cup olive oil

Preparation Steps:
Whisk all ingredients together; serve over fish.

Notes:
Demetrios speculates that these ingredients might approximate a Byzantine mustard sauce. This is very thin, a vinaigrette. I made it a bit “thicker” by increasing the mustard from 1 teaspoon to 3 tablespoons. The result is a vinaigrette that remarkably retains an emulsion for a very long time. Must be something in the mustard seed. The extra mustard gives the sauce quite a bit of heat, which tends to grow upon the tongue as the vinegar is washed away.
Beware!
Demitrios says that the mustard sauce to which Hierophile refers might also be something like a modern Hollandaise sauce.