In a saute pan, over high heat, heat the olive oil. Add the onions and saute until caramelized, about 3 to 4 minutes. Add the garlic, parsley, and rice wine vinegar. Saute for 1 to 2 minutes. Season with salt and pepper.
In a small mixing bowl, combine the peppercorns, thyme, bay leaves, cloves, garlic, and whole juniper berries. In a saucepan, over medium heat, combine the water, brown sugar and salt. Bring to a boil and stir to dissolve the sugar and salt. Remove from the heat and add dry spice mixture and steep for 1 hour. Place the duck breast in a glass or plastic container. Pour the seasoned brine to cover the breasts completely. Cover and refrigerate for 48 hours, turning the breasts a couple of times. Remove the duck breasts from the brine and rinse thoroughly with cool water. Pat dry with a towel. Preheat the oven to 250 degrees. Combine the crushed juniper berries and ground black pepper in a small bowl. Using the palm and heel of your hands, press 2/3 of the berry and pepper mixture into the underside of the breasts. Press the remaining mixture onto the skin side. Place the breasts, skin side down, on a rack in a roasting pan and roast for 1 hour. Remove and let cool for 30 minutes. Wrap the breasts tightly in plastic wrap and place in an airtight container. Store in the refrigerator for at least 1 week before using. To serve, remove the meat and slice thin. Slice the french bread in 1/2 horizontally. Smear the Creole mustard on cut sides of the bread. Lay the slices of the duck on one side of the bread. Lay the slices of cheese over the duck. Top the cheese with the onion marmalade.
Slice the sandwich and serve.