Place the bread on a rimmed baking sheet and drizzle with olive oil and salt. Toast for 5 to 8 minutes, until crisp. Transfer to a large bowl.
In a heavy-bottomed skillet over medium-high heat, heat the butter until it's melted and foamy. Add the onion, carrot, celery and garlic and cook, stirring, until the vegetables are tender, about 10 minutes. Stir in the parsley, sage and chicken stock. Season with salt and pepper. Add the toasted bread and toss to combine. If you need more liquid for the bread, add more chicken stock 1/4 cup at a time. Remove pan from heat and allow to cool to room temperature
Preheat the oven to 350 degrees F.
Once the stuffing is cool, wear kitchen rubber gloves to stuff the cavity of the quails with about 1 cup of stuffing per quail. Place the remaining stuffing on the bottom of a roasting pan or large cast-iron skillet. Season the quail with salt and pepper and rest on top of the stuffing. Roast the quail for 15 to 20 minutes or until juices run clear when you pierce the skin. Serve right away.