Prepare the pie dough. In a large bowl, whisk flour and salt to combine. Add cubed butter, tossing until each cube is coated in flour.
Use your fingertips or heels of your hands to press each cube of butter until it’s nearly flat – making shingles of butter. Repeat with each butter piece one time, but do not keep working shingled pieces to make them smaller.
Add water and mix gently to combine. Continue to add water 1-2 tablespoons at a time, just until dough is evenly hydrated. It should be homogeneous (no dry pockets of flour) but not sticky. Sticky dough is the result of too much water.
Form dough into a 1 inch-thick disc, wrap tightly in plastic wrap, and refrigerate 45-60 minutes.
On lightly floured surface, roll out dough into a square ½ inch thick. Sprinkle ¼ (approx. 2 ounces) of Kaltbach™ Le Gruyère® evenly over dough, and fold it into quarters. Repeat this folding process, using another ¼ of the cheese. Wrap dough in plastic wrap and refrigerate 45 minutes.
Repeat step 5, then re-wrap dough and refrigerate 30 minutes.
On lightly floured surface, roll out dough to ¼ inch thick. Cut dough into squares
4 x 4 inches wide. You can gently knead scraps of dough back together and re-roll one additional time, if needed. You should end up with 12 squares.
Heat oven to 425°F. Line two baking sheets with parchment paper, and arrange dough squares evenly between them.
Prepare the tart filling. In a medium bowl, toss peach halves with honey to combine. Place a few slices of onion on the base of each piece of dough, then top with a peaches. Sprinkle pinch of Aleppo pepper over each pastry.
In a small bowl, stir rosemary and salt to combine. Add a small pinch to each pastry before baking. (You can add more to taste after baking as well.)
Bake. Bake pastries until crust is very golden brown and peaches are very tender, 17-20 minutes.
Transfer pastries to cooling rack, let cool 5-10 minutes before serving.