Spray 10-inch skillet with cooking spray. Heat over medium-high heat until hot. Add turkey; cook until no longer pink. Stir in thyme and pepper.
In large bowl, mix contents of sauce packet from potato mix, potato slices and butter. Add boiling water; stir until butter melts. Add milk; mix well.
Stir in turkey, bell pepper and onion. Spoon mixture into 3 1/2- to 4-quart slow cooker. With back of spoon, press down potatoes until covered with sauce.
Cover; cook on Low heat setting at least 7 hours.