Brown sausage and onion in skillet over medium-high heat until sausage is browned. Drain fat; mix sage, 1/2 teaspoon maple flavoring and 1/2 teaspoon vanilla into the meat. Set aside.

Heat corn syrup, butter and brown sugar in a saucepan over medium heat until butter is melted and sugar is dissolved. Pour mixture into an 8-inch square baking dish. Place 4 slices of bread in a single layer over the syrup, top with the sausage, then place the remaining 4 slices of bread on top of sausage.

Whisk eggs, milk, 2 teaspoons vanilla, 1 teaspoon maple flavoring and the salt. Carefully pour over bread.

Cover; refrigerate 8 to 24 hours.

Preheat oven to 350º F. Bake uncovered for 35 to 45 minutes. Remove from oven; let stand 5 to 10 minutes. Cut into squares. Invert each piece when serving.