In nonstick 10-inch skillet, melt 1 tablespoon margarine over medium-high heat. Add turkey to skillet; sprinkle with salt and pepper. Scatter onion around turkey. Cook turkey and onion 6 to 7 minutes or until turkey is browned on both sides.
Meanwhile, in 4 1/2- to 6-quart slow-cooker pot, place thyme, celery, carrots, and potatoes.
Transfer turkey and onion to slow cooker. Melt remaining 2 tablespoons margarine in same skillet over medium heat. Add flour and cook 1 minute, stirring. Add broth and cream; heat to boiling, stirring constantly. Pour broth mixture into slow cooker. Cover slow cooker with lid and cook on low setting as manufacturer directs, 7 to 8 hours or until turkey and vegetables are very tender.
To serve, discard thyme sprigs. With tongs, transfer turkey to cutting board. Cut meat from bone into bite-size pieces; return meat to slow cooker. Cover slow cooker and heat through on high setting if necessary.