Combine flour, butter, salt, and pepper in a food processor. Pulse into coarse crumbles.
Add cream and water. Pulse until a dough forms and pulls away from the walls of the food processor.
Form into a flat disc, cover with plastic wrap, and chill in the fridge for at least 30 minutes.
Steam potatoes until just starting to tenderize – a sharp knife can be inserted. Set aside to cool.
Melt 1 tbsp butter in a pan on medium-high. Sauté onions until caramelized, stirring frequently, about 10-15 minutes depending on size of pan (it is a lot of onions).
Add crème fraiche, thyme, and salt to onions, tossing to combine. Set aside.
Preheat oven to 375F.
Roll out dough on a lightly flour surface. Roll to ¼” thick and about 18” around.
Assemble on a lined baking sheet: Place dough on sheet. Place potato rounds in a 9” circle on the center of the dough disc. Reserve 3-4 rounds for the top of the galette (optional).
Spoon the onion mixture on top of the potatoes, spreading evenly.
Fold the bare edges of the dough over the filling, giving the galette five or six edges.
Place the remaining potato slices in any desired pattern on top of the onion mix if desired.
Whisk water and egg together. Use a pastry brush to glaze the exterior of the dough and drizzle the remaining egg on the filling.
Bake for 35-45 minutes until the crust is golden and the filling is bubbling slightly.
Allow to cool for 10-15 minutes before serving. Also does well at room temperature.