For the dressing:
To make the dressing, combine the yolk with the vinegar, Dijon, brown sugar, and seasonings in the bowl of a food processor. In a separate bowl, mix the dry mustard with a few drops of water to turn it into a smooth paste; add to the vinegar mixture. Blend until sugar is dissolved and ingredients are well-incorporated. With the machine running, pour the oil in a slow stream through the feeding tube until emulsified. Makes 1 cup.
Heat 2 inches of oil over medium heat in a heavy-bottomed saucepan or skillet until hot but not smoking.
Meanwhile, combine the flour and salt in a small bowl. Whisk the egg with the water and hot sauce in another small bowl. Toss each oyster in flour mixture, patting off any excess; coat with egg wash and dredge in the cornmeal. Cook in batches, dropping breaded oysters into oil and frying until golden brown, about 2 minutes. Drain on paper towels. Keep warm.
Combine the Romaine, peas, and onion in a serving bowl. Toss desired amount of dressing with greens. Divide among serving plates and top each salad with fried oysters
