Preheat oven to 375°F. Spray a casserole or baking dish with cooking spray. In a large pot or Dutch oven, heat the olive oil and butter over medium high heat. Add the celery and cook for 2 to 3 minutes until slightly softened. Add the onions, carrots and garlic. Cook an additional 2 to 3 minutes. Whisk flour into the vegetable mixture. Cook about 1 minute to remove the raw flour taste. Whisk in chicken stock and red-hot sauce. Continue whisking until sauce comes to a boil and has thickened like a gravy (if sauce is too thick, add a little more chicken stock).
TIP: You can substitute 2 tablespoons of olive oil for the 2 tablespoons of butter.