In a deep, heavy skillet, heat the olive oil and saute the chives for 2 minutes. Add the mussels or shrimps, and cook for 1 minute. Pour in the wine, and when it starts to boil, add the mustard, chili pepper, half the parsley, and the water or stock. Cook for about 3 minutes, then add the lemon juice, feta cheese, and the rest of the parsley. Do not stir, but shake the pan to distribute the ingredients evenly.
Remove from the heat, taste, and add salt if needed. Let cool to just warm, and serve. It can also be refrigerated for 2 to 3 days.