Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper. If using puff pastry, unroll it onto the prepared baking sheet. If using homemade dough, roll it out into a 12-14 inch circle and transfer to the baking sheet.
Step 2: Prepare the caramelized onions. Heat 1 tablespoon of olive oil and the butter in a large skillet over medium-low heat. Add the sliced onions, 1/4 teaspoon of salt, and 1/8 teaspoon of pepper. Cook, stirring occasionally, for 30-45 minutes until the onions are deeply golden brown and very soft. Remove from heat and let cool slightly.
Step 3: Prepare the potatoes. In a medium bowl, toss the thinly sliced potatoes with the remaining 1 tablespoon of olive oil, minced garlic, chopped fresh thyme, 1/4 teaspoon of salt, and 1/8 teaspoon of black pepper. Ensure all slices are lightly coated.
Step 4: Assemble the galette. Spread the cooled caramelized onions evenly over the pastry dough, leaving a 2-inch border around the edges clean. This border will be folded over to form the crust.
Step 5: Arrange the potatoes. Shingle the seasoned potato slices over the caramelized onions in concentric circles, slightly overlapping them. Try to create an appealing pattern.
Step 6: Add the cheese. Evenly sprinkle the grated sharp cheddar cheese over the potato layer. Don’t be shy; the cheese adds crucial flavor and helps bind the filling.
Step 7: Fold the edges. Carefully fold the 2-inch pastry border up over the filling, crimping it gently every few inches to create a rustic, free-form crust. The center of the galette with the filling will remain exposed.
Step 8: Apply egg wash. In a small bowl, whisk the egg with 1 tablespoon of water to create an egg wash. Brush the exposed pastry edges thoroughly with the egg wash for a golden, shiny crust.
Step 9: Bake the galette for 35-45 minutes, or until the crust is deeply golden brown and the potatoes are tender when pierced with a fork. If the crust browns too quickly, you can loosely tent it with foil.
Step 10: Cool and serve. Remove the Cheddar, Potato & Caramelized Onion Galette from the oven and let it cool on the baking sheet for 10-15 minutes before carefully sliding it onto a cutting board. Slice into wedges and serve warm. Enjoy!