1. Heat oven to 400 degrees F.
2. In a large bowl, with a fork, toss together piecrust mix, 1/2 cup of the cheese and the apple juice; shape into a ball. Roll out on a floured surface to 14-inch circle. Roll pastry back onto rolling pin; unroll onto a large baking sheet.
3. In a large bowl, toss together 1/2 cup of the shredded cheddar, apples, sugar, cornstarch and nutmeg. Set aside.
4. Sprinkle remaining 1/2 cup cheddar cheese onto center of crust, leaving a 3-inch border. Mound apple mixture on top of cheese.
5. Fold edge of crust up and partway over filling. Brush crust with egg yolkwater mixture. Bake galette at 400 degrees F for 35 to 40 minutes until crust is lightly brown and apples are tender. Cut into wedges and serve slightly warm or at room temperature.