Rub the duck breasts with kosher salt and cracked peppercorns, wrap them in plastic wrap, and refrigerate for 24 hours.
Remove the duck breasts fro the refrigerator, rinse off the salt and pepper, dry them, then put them in a small glass dish and spread them with the syrup, garlic cloves, juniper berries, and whole peppercorns. Cover and refrigerate for 2 days.
Remove the breasts from the marinade.
Preheat the oven to 400ºF.
Season the duck breasts with salt and pepper. Heat 1 teaspoon of the olive oil in a large ovenproof sauté pan over medium heat. Place the breasts, skin side down, in the pan and cook until the skin is crispy and brown, 10 minutes. Turn the breasts over and cook 10 minutes more. Transfer the pan to the oven. Roast for about 5 minutes for medium-rare; 8 minutes for medium; 10 minutes for well done. Remove from the oven and let the meat rest for about 2 minutes, then cut diagonally into 1/4-inch-thick slices. Set aside and keep warm.
In a large sauté pan, heat the remaining 1 tablespoon olive oil over medium heat. Add the red onions, season with salt and pepper, and cook, stirring, until slightly wilted, 1 to 2 minutes. Add the cabbages, spinach, green onions, and parsley and season with salt and pepper. Cook, stirring, for about 1 minute, just to warm the mixture slightly. Transfer the mixture to a large mixing bowl. Add the mustard and 1/4 cup of the aioli mayonnaise and toss to mix thoroughly. Set aside.
Spread 1 side of all the slices of bread generously with the remaining 1/2 cup aioli mayonnaise. Arrange the duck equally on 4 slices of the bread. Top each with 3 slices of Brie, then 3 slices of bacon, and equal amounts of the slaw. Top with the remaining slices of bread.
To serve, cut the sandwiches in half diagonally.
Yield: 4 servings