In a large saucepan brown meat, half at a time, in hot oil; drain fat. Return all meat to pan. Stir in broth, wine, dried thyme (if using), garlic, and bay leaf. Bring to boiling; reduce heat. Simmer, covered, for 20 minutes.
Stir in squash, parsnip, sweet potato, celery, and onion. Return to boiling; reduce heat. Simmer, covered, about 30 minutes more or until meat and vegetables are tender. Discard bay leaf.
In a small bowl combine yogurt, fresh thyme (if using), and flour. Stir 1/2 cup of the hot liquid into the yogurt mixture. Add yogurt mixture to saucepan. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Season to taste with salt and pepper.