Preheat oven to 450 degrees F.
In a 10 to 12-inch saute pan, heat olive oil until smoking. Add onion and cook until softened (about 8 to 10 minutes). Add apple and grapes and cook until softened (about 6 to 7 minutes). Allow to cool. Break up sausage into large mixing bowl. Add rosemary, sage, and peppercorns and mix well. Add apple mixture and mix well with hands. Place half of cool stuffing mixture in each neck and tie securely. Place on baking sheet, prick several times with fork and bake 20 to 25 minutes until crisp and golden.
Meanwhile, place Dijon mustard, prune vinegar, and remaining grapes in blender. Process until grapes are broken but still lumpy. Add oil and process 1 minute more, season with salt and pepper, and set aside.
To assemble, slice a wedge of fritatta and place on a plate. Slice warm duck neck on bias into 8 slices, place each slice over a fritatta, drizzle with vinaigrette and garnish with cracklings and serve.