In 4- or 5-quart Dutch oven, melt butter over medium heat. Add onion; cook 3 to 4 minutes, stirring occasionally, until tender. Add broth, cream cheese, salt and pepper; cook until cream cheese has melted and broth is boiling, stirring occasionally. Increase heat to medium-high; stir in turkey and mixed vegetables. Heat to a simmer; reduce heat to medium-low. Cook until thoroughly heated.