1. Rinse thighs and place in the bottom of a 4 qt slow cooker. Add onion. In a bowl, combine juice, mustard, and red pepper. Pour over turkey.
2. Cover and cook on low setting for 5-6 hours or on high for 3 hours. Use meat thermometer to make sure turkey is at least 165F. Remove turkey and cover to keep warm.
3. For sauce, strain cooking juices into a measuring cup. You should have 1 1/2 to 2 cups. If you are short, add water to make 1 1/2 cups. Pour juices into a saucepan and heat on medium. Stir together cornstarch and water, add to mixture in pan. Cook and stir over medium heat until thickened and bubbly. Add poultry seasoning and taste, then add salt and pepper to taste. If sauce is too tart, add a little bit of sugar.
To serve, cut or shred turkey from thighs and portion onto hot cooked rice. Top with sauce.