Meanwhile, in 4-quart Dutch oven, cook bacon over medium-high heat until crisp. Remove bacon with slotted spoon; set aside. Decrease heat to medium. Add onion. Cook 3 to 4 minutes, stirring occasionally, until tender. Add 3 tablespoons of the flour, the thyme leaves and pepper; cook 1 to 2 minutes, stirring occasionally. Beat in chicken stock and mashed potatoes with whisk; heat to boiling. Reduce heat to low; simmer 30 minutes. Stir in cream; heat to simmering. In medium bowl, toss 1 1/2 cups of the cheese with remaining 1 tablespoon flour. Add cheese mixture, 1/2 cup at a time, until cheese is melted and soup is smooth after each addition.