Remove sausage meat from casings and cook in large skillet over medium heat. Once meat has browned, add onions and cook until translucent. Add butter, cabbage, apple, chicken stock, thyme leaves, salt, and pepper and cook for 15-20 minutes or until cabbage has cooked down and stock reduced fully. Remove from heat.
Pat quail with paper towel to remove moisture. Drizzle the skin and interior of quail with olive oil and sprinkle with salt and pepper. Lay quail flat with skin-side down. Spoon stuffing onto the two breast area. Wrap quail around stuffing. Truss using butcher twine. Place quail breast-side up on a parchment paper-lined baking sheet. Bake for 15-20 minutes or until golden and crispy.
For the plums, whisk wine and sugar together in a saucepan. Add juniper berries and thyme and heat to a gentle boil. Turn heat down to low, add plums, and simmer for 15-20 minutes. Remove plums and reduce sauce for three to four minutes. Remove from heat. Divide poached plums between serving dishes, then top each dish with one stuffed quail.