Prepare pastry by stirring flour, sugar and salt in a large bowl until evenly mixed. Using your fingers and thumbs, cut butter into flour mixture until pea-sized crumbs form. Mix in lemon juice and 4tbsp. of ice cold water. Although the texture may appear sandy, check if you can press it into a dough. If needed, add more ice-cold water 1 tbsp.at a time until it can be pressed together. Be careful not to add too much water or overmix. Turn into a large bowl or onto a surface. Form into dough, then into two discs. Wrap with plastic wrap. Refridgerate until firm, at least 1 hour or up to 3 days.
Remove pastry from fridge. Let stand for 15 minutes or until rollable. Meanwhile, peel and core apples. Chop into small pieces. Place in a medium bowl. Drizzle with lemon juice. Sprinkle with brown sugar, chopped sage, flour and salt. Toss to coat. Sprinkle in cheese and toss to coat.
When pastry is ready, preheat oven to 375F (190C)
Roll one disc out on a lightly floured surface until about 1/4 inch thick. Cut pastry into 4 circles using a lightly floured, 4 1/2inch (11cm) glass or cookie/scone cutter. Set aside. Bring dough back together and roll out again to cut out another circle. Repeat with remaining disc. Arrange circles over counter. Brush around edge of each circle with beaten egg. Top 1 side of each with a scant 1/4 cup of apple filling, leaving edges uncovered. Fold uncovered side of pastry over filling. Using a lightly floured fork, press edges together. Arrange over a large parchment lined or greased baking sheet. Lightly brush tops with egg.
Bake in center of oven for 20 minutes or until pastry is lightly golden. Cool for at least 10 minutes. Serve warm or at room temperature. Pies are best enjoyed the day they're made.