Prepare an outdoor grill for high heat.

Poke sausages several times with a fork. Place in a large 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 large 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, 10 to 15 minutes. Season with salt and pepper.

Meanwhile, place boiled sausages on the preheated grill (or under the broiler) and cook until well browned. Serve on a mound of caramelized apples and onions.