Preheat oven to 350°. In a large skillet over medium heat, cook and crumble sausage until no longer pink, 5-7 minutes; drain. Mix biscuit mix, milk, eggs and oil until blended; stir in sausage. Transfer biscuit mixture to a greased 13x9-in. baking dish. Top with apples; sprinkle with cinnamon sugar. Bake until set, 30-45 minutes. Serve with syrup.
To make ahead: Refrigerate, covered, several hours or overnight. To use, preheat oven to 350°. Remove casserole from refrigerator; uncover and let stand while oven heats. Bake as directed, increasing time as necessary until a knife inserted in the center comes out clean.