If you have a food processor, it would be best to use it for the first part of the crust process. If not, use a pastry cutter or long fork. Mix flour, sugar and butter together until combined well. It doesn't have to be totally smooth, getting the chunks down to pea size is good enough.
Transfer to a medium bowl. Add water a tablespoon or two at a time until you have a nice consistency. You want the dough to hold together but not be overly flaky. You might not need to add a full 1/2 cup of water, judge this as you go. If your dough is holding together well and isn't flaky, stop. Transfer dough to a plastic bag and refrigerate for an hour or so.
Preheat oven to 400F°. Put dough between two pieces of wax paper. Roll out the dough into a large enough circle to fit your tart pan. Prepare your tart pan with butter or Pam for baking. Transfer your dough to the tart pan and toss in the freezer for 10 minutes or so. Butter or spray tin foil, fitting it to the dough. Cook for 25 minutes. Remove foil, brush egg over your crust and bake for an additional 5 minutes. Cool completely.
In a small bowl mix together sugar and cinnamon. When your crust has cooled, sprinkle 2 tbsp cinnamon sugar mixture over the crust. Now, working as quickly and safely as possible. One at a time... or all at once, whichever you prefer, slice an apple in half, use a melon baller to remove the core. Peel the skin from your apple and slice each half into thin pieces. Arrange slices on your crust. Repeat with the remaining apples. Sprinkle the remaining cinnamon sugar mixture over the top of the apples. Top off with slices of butter. Bake for 35 minutes. Let cool slightly before slicing into.
Apple Tart - https://www.bakedbyrachel.com/apple-tart/