In a food processor or a blender, process the olives, shallots, lemon juice, mustard, and zest to a coarse puree. With the machine on, add the olive oil in a slow steady stream through the feeder tube or blender top. Season to taste with salt and pepper. Allow the flavors to meld together for at least ½ hour before serving. The sauce will keep for about 5 days, refrigerated.