With a fork, poke sausages several times. Place in a pot, cover with water, and simmer over medium-high heat until cooked through, about 7 minutes. Remove from heat, and set aside.
Warm butter in a skillet over medium heat. Stir in onions, and cook until soft and translucent. Stir in apples, vinegar, and brown sugar; cook, stirring gently, until caramelized, about 10 to 15 minutes. Season with salt and pepper.
Meanwhile, place sausages on grill (or under broiler), and cook until well browned. Serve on a mound of caramelized apples and onions.
