Rustic Cheddar Mashed Potatoes

2 lbs. russet potatoes, peeled and diced
1 c. water
1/3 c. butter, cut in small pieces
1/2 - 3/4 c. milk
1 1/4 tsp. salt
1/2 tsp. black pepper
1/2 c. finely chopped green onions
2-3 oz. shredded cheddar cheese

Combine potatoes and water in slow cooker; dot with butter. Cover and cook on High for 3 hours or on Low for 6 hours. Using an electric mixer on medium setting, whip potatoes until well blended. Add milk, salt, and pepper and whip until well blended. Stir in green onions and cheese; cover tightly and let stand 15 minutes to allow flavors to blend and cheese to melt.