In a food processor combine flour, sugar, baking powder, pepper, and the 1 teaspoon salt; cover and pulse with a few on/off turns to combine. Add the 1 1/2 cups butter; cover and pulse with several on/off turns until the largest pieces of butter are no bigger than a small pea (about eight 1-second pulses). Add cheese; cover and pulse with a few on/off turns to work it in. Add egg yolks; cover and pulse with a few on/off turns to combine.
Transfer the mixture to a very large bowl. Gradually add 1/2 cup of the ice water, stirring with a fork until combined. Squeeze a small handful of the dough together (it should easily hold together without crumbling apart). If necessary, add enough of the remaining 1/4 cup ice water, 1 tablespoon at a time, until dough holds together, stirring between additions. Gather mixture into a ball, kneading gently until it holds together. Transfer to a cutting board; cut into six equal portions. Place each dough portion on a sheet of plastic wrap; fold the plastic wrap tightly over the ball. Pat each plastic-wrapped dough ball into a 4-inch disk. Chill for 1 hour to 2 days.
For the galettes, place apple slices in a large bowl. Add the 1/4 cup melted butter, the apple jelly, and lemon juice; toss to combine. Line two extra-large baking sheets with parchment paper; set aside.
On a well-floured surface, roll a pastry disk to a 9- to 10-inch circle, about 1/4-inch thick. Spread 1 1/2 teaspoons of the mustard over dough circle. Overlap two slices of the ham in the center of the dough circle; arrange one-sixth of the apple mixture over the ham, making sure to leave a 3-inch border uncovered. Fold dough edges over apple slices, loosely pleating the dough and leaving apple slices uncovered in the center. Using a spatula, carefully transfer the galette to a prepared baking sheet. Repeat with the remaining dough disks, mustard, ham, and apple mixture, placing three galettes on each baking sheet. Cover and chill for 1 hour.
Arrange one oven rack in the upper-middle position and another oven rack in the lower-middle position of the oven. Preheat oven to 375 degrees F.
In a small bowl beat together egg, the 1 tablespoon water, and the pinch salt. Using a pastry brush, brush the egg mixture over the edges of each galette. Bake in the preheated oven for 35 to 40 minutes or until the dough is golden and the apples are tender. Cool slightly on wire racks. Serve warm.