Preheat oven to 425° and set the oven rack in the middle of the oven.
Peel and slice apples (slices should be thin, but need not be uniform). If you have a food processor, use the slicing blade to speed up the process.
Place apple slices in a large bowl. Add lemon juice. In a small bowl, mix brown sugar, salt, cinnamon, nutmeg, and flour until well-combined. Pour the dry mixture over the apples and, using your hands, toss until all slices are fairly evenly coated.
Place one pie crust on a 9-inch pie plate and add half of the apple mixture. Place four slices of butter (spaced evenly) on top of the apples. Add the remaining apple mixture arranging the slices so they form a small mound. Top with remaining butter.
Place the second pie crust over the filling. Seal the edges of both of the crusts by pressing together, and then tucking them under to form an even edge.
Use your hands to form a decorative scalloped edge if desired. With a sharp paring knife, make four 1-inch to 2-inch slits in the top pastry. The slits should form a cross shape but without meeting in the center.
If you want to add decorations using the leftover dough, use cookie cutters to cut out shapes. Place the cut-out shapes around the edges or over the top of the pie. These can be used instead of or in addition to a second pie crust.
Brush with egg wash if desired. (If you are planning to freeze your pie, add the egg wash later, as instructed below, right before baking.)
Cover the edges of the pie crust with a pie shield. (If you do not have a pie shield, no worries. It helps but is not essential.) In addition to the pie shield or instead of the pie shield if not using one, gently lay a piece of foil over the top of the entire pie — this will protect the top crust (or lattice or decorations) from burning. Bake pie at 425 degrees for 20 minutes on a rack set in the middle of the oven.
Remove pie from oven and remove foil laying over top (but leave pie shield if using one). Reduce heat to 375 degrees and bake the pie for 30 more minutes.
Remove the pie from oven and sprinkle with sugar. Return to the oven and bake for 10 more minutes or until apples are tender when a toothpick is inserted through one of the slits.
Cool for 1-2 hours before serving.