Preheat oven to 350 degrees F (175 degrees C). Grease a 9x12-inch baking dish.
Heat vegetable oil in a skillet over medium heat, and cook the garlic and onion until the onion is translucent, about 5 minutes; stir in the jalapeno pepper and flour. Stir a few times to mix in the flour. Stir in the chicken stock, about 1/4 cup at a time; cook until thickened, stirring constantly, about 5 minutes. Mix in the tomatoes and bring the mixture to a simmer.
While the sauce is simmering, combine sour cream, 1/2 cup of Cheddar cheese, and turkey in a bowl. Place a tortilla down onto a work surface, and spoon 1/4 of the turkey mixture onto the tortilla in a line down the center. Roll the tortilla around the filling; place into the prepared baking dish with seam side down. Repeat with the other tortillas. Pour the tomato sauce over the filled tortillas. Sprinkle the casserole with black olives and 1 tablespoon of Cheddar cheese as garnish.
Bake uncovered in the preheated oven until the cheese topping is browned and the casserole is bubbling, about 30 minutes.