Heat the butter in a nonstick skillet. Add the onions and msuhrooms and sauté until the onions are beginning to soften. Add half of the onion and mushroom mixture to a 4 quart slow cooker. Add the turkey. Sprinkle with sage, salt, and pepper. Top with the remaining onion and mushroom mixture.

Add the water. Cook on high for 2-3 hours or on low for 6-8 hours.

Make sure that your slow cooker is at least half filled before cooking for best results.