In a mixing bowl, stir together the rabbit and pancetta with your hands until well mixed. Add pepper, salt and white wine and mix until well blended, again with your hands. You must move quickly here or your body temperature could change the texture of the fat.
Set up the sausage stuffer and place the casing over the funnel feeder. If you do not have a sausage stuffer, form the sausage into 1/4-pound logs by hand, and be careful handling them. Stuff the sausage into the casings, twisting every 3 1/2 to 4 inches so that each sausage is about 4 ounces, and form 10 to 12 sausages. Prick them all over with a needle and set aside in a cool place until ready to cook. In a Dutch oven, combine the olive oil and butter and cook over medium heat 1 minute. Add the onion slices, reduce the heat to low and cook, covered, until onions are very soft and browned, or caramelized.
Stir in the balsamic vinegar and tomato paste and cook 5 minutes over low heat. Grill the sausages until just cooked through, about 6 minutes to the side. Remove them and place in the pot with the onion mixture. Simmer, covered, for 10 minutes and serve immediately.