2. In a blender, combine the eggs, flour, milk, yogurt, vanilla, cinnamon and salt. Blend until very smooth, then set aside. Alternatively, the batter can be made in a bowl using a whisk.
3. Coat a large cast-iron or oven-safe nonstick skillet with cooking spray, then heat it over medium-high. Add the sausage and cook, breaking up the meat, until just starting the brown, about 10 minutes.
4. Meanwhile, peel and core 2 apples, then cut them into 1/2-inch chunks. Add the cut apples and onion to the skillet, then saute until they are just tender, about 6 to 8 minutes. Spread the sausage-apple mixture evenly over the bottom of the skillet and remove the pan from the heat.
5.Core the remaining apple then cut it crosswise into very thin slices. A mandoline is best for this, but a food processor or careful knife work is fine, too.
6. Pour the batter evenly over the sausage-apple mixture. Tilt the pan as needed to ensure it spreads evenly in the pan. Arrange the apple slices over the top of the batter; it's fine if they overlap. Sprinkle the apples with the sugar, then bake for 20 minutes.
Arrange the apple slices over the top of the batter; it’s fine if they overlap. Sprinkle the apples with the sugar, then bake for 20 minutes.
6. To serve, slice into wedges and drizzle with maple syrup.