Heat olive oil over medium heat in a medium saucepan. Add onion, lightly sprinkle with salt to sweat liquid and keep from browning; cook and stir 2 to 3 minutes until softened and translucent. Add carrots, celery, and potato; cook and stir 2 minutes more. Add broth and sage; bring to boil. Reduce heat and simmer 6 minutes or until vegetables are nearly tender.
Meanwhile, make a roux: melt butter in a small saucepan over medium heat. Once butter melts, add flour all at once. Whisk constantly for a minute or two, just to cook out the raw flour taste.
Add cooked turkey and roux to broth mixture and stir to combine; bring to simmer. Cook and stir until thickened, about 3 minutes. Stir in half and half and season to taste with salt and freshly ground black pepper. Serve.