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 mixture to broth mixture all at once and stir until smooth.
Add cooked turkey and peas 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.