Preheat the broiler. In a large skillet with a tight-fitting lid, heat the EVOO and 3 tbsp. butter over medium-high heat. Add the celery, carrots, onion, bay leaf and thyme; season with salt and pepper. Partially cover the pan to sweat the vegetables, 7 to 8 minutes. Sprinkle the veggies with the flour and stir for 1 minute. Whisk in 2 1/2 cups chicken stock and cook until thickened. Add the turkey and heat through. Lower the heat to a simmer.
Meanwhile, simmer the remaining 1 1/2 cups stock and the milk. Whisk in the polenta to thicken, 2 to 3 minutes. Stir in the cranberries (or sauce) and orange zest, then the remaining 2 tbsp. butter and half of the cheddar.
Spread the polenta over the turkey mixture; top with the remaining cheddar. Broil for 1 to 2 minutes.