Brush garlic with a little of the oil and roast it in aluminum foil at 375 F for 20 minutes. Squeeze garlic flesh from skins and blend with oil, mustard, lemon juice, and seasonings until smooth, making a thick mayonnaise.
Poach cod in the wine in a large covered skillet over moderate heat for 6 to 10 minutes, or until cod is just fork-tender. Remove cod to a serving platter. Thin garlic sauce in blender by adding 1/4 cup of poaching liquid. Pour sauce over the cod. Garnish with fresh parsley.