Preheat oven to 350°F. Grease a 9"x 12" baking dish. Heat vegetable oil in a skillet over medium heat, and cook garlic and onion until the onion is translucent, about 5 min.; stir in jalapeño pepper and flour. Stir a few times to mix in the flour. Stir in chicken stock, about 1/4 cup at a time; cook until thickened, stirring constantly, about 5 min. Mix in tomatoes and bring the mixture to a simmer. While the sauce is simmering, combine sour cream, cheddar cheese and turkey in a bowl. Place a tortilla onto a work surface, and spoon 1/4 of the turkey mixture onto the tortilla in a line down the centre. Roll the tortilla around the filling; place in prepared baking dish seam side down. Repeat with the other tortillas. Pour tomato sauce over the filled tortillas. Sprinkle casserole with black olives and 1 tbsp cheddar cheese. Bake uncovered until cheese topping is browned and casserole is bubbling, about 30 min.