Caramelized Onion and Gruyère Potato Galette Step-by-step instructions

Before you start

Before starting, set the oven to 400°F (200°C) and place the rack in the center. Slice the onions thinly and begin caramelizing them — this takes about 15 minutes, so start while you prep the potatoes.

Scrub the potatoes thoroughly; no need to peel if you like the rustic look. Set up your mandoline with a 1.5 mm blade.

Have the parchment paper ready to line the skillet. Grate the Gruyère and pick the thyme leaves. Set everything within reach so assembly goes quickly.

Instructions

Prep and Caramelize

1. 1Caramelize the onions
   Heat a cast-iron skillet over medium heat and add a small knob of butter. Add the sliced onions and cook, stirring occasionally, until they turn deep golden brown and sweet, about 15 minutes. Pour in the balsamic vinegar and stir to scrape up any browned bits. Cook until the vinegar is absorbed and the onions are jammy. Transfer to a plate and let cool slightly. Wipe the skillet clean.
   Look for Onions are soft, deeply browned, and the vinegar has reduced to a glossy glaze.
   Tip For even caramelization, keep the heat medium-low and stir only every few minutes.
2. 2Slice the potatoes
   Using a mandoline, slice the potatoes into 1.5 mm thick rounds. Keep them in a bowl of cold water to prevent browning while you work. Drain and pat thoroughly dry with a clean kitchen towel before assembling.
   Look for Potato slices are thin, uniform, and dry.
   Tip A mandoline ensures even slices; if using a knife, aim for paper-thin and as consistent as possible.
3. 3Prep the butter
   Melt the remaining butter and stir in the fresh thyme leaves and a pinch of flaky salt. Set aside.
   Look for Butter is melted and fragrant with thyme.
   Tip Infuse the butter for a few minutes to deepen the herb flavor.

The onions should be soft, deeply browned, and glossy from the balsamic vinegar. The potatoes are sliced paper-thin and dried. The butter is melted and infused with thyme. Everything is ready for assembly.

Assemble and Bake

4. 4Assemble the base
   Line the cleaned cast-iron skillet with a circle of parchment paper. Spread the caramelized onions evenly over the bottom, then sprinkle with half of the grated Gruyère.
   Look for Onions and cheese form a solid, even layer.
   Tip Make sure the parchment fits snugly so the galette unmolds cleanly later.
5. 5Shingle the potatoes
   Arrange the potato slices in concentric circles over the onions and cheese, overlapping them slightly like shingles. Start from the outer edge and work inward. Brush the potatoes generously with the thyme butter, then sprinkle with the remaining Gruyère and a pinch of flaky salt.
   Look for Potatoes cover the entire surface in neat, overlapping rows.
   Tip Keep the shingles tight, but don't be afraid to let them overlap naturally for a rustic look.
6. 6Bake the galette
   Place the skillet in a preheated 400°F (200°C) oven and bake for 45 minutes, until the top potato slices are deeply golden and crisp at the edges, and the bottom is bubbling and caramelized.
   Look for Top is golden brown with crispy edges; the cheese is melted and bubbling around the sides.
   Tip If the top browns too quickly, cover loosely with foil for the last 10 minutes.

The galette is assembled with concentric potato shingles, brushed with butter, and topped with cheese. It bakes until the top is golden and crisp, and the kitchen smells of roasted onions and thyme.

Unmold and Serve

7. 7Unmold and serve
   Remove the skillet from the oven and let cool for 5 minutes. Run a knife around the edge, then carefully invert the galette onto a serving platter. Peel off the parchment. Garnish with fresh thyme sprigs and a sprinkle of flaky salt. Slice into wedges and serve warm.
   Look for The galette releases cleanly, revealing the caramelized onion and cheese bottom.
   Tip Use a large spatula to support the galette when flipping to avoid breaking it.

After a short rest, the galette is inverted onto a platter, revealing the caramelized onion and cheese crust. It slices cleanly into wedges with a loud crunch. Garnish with fresh thyme and flaky salt before serving.

Common mistakes

The most common mistake is slicing potatoes too thick, which results in uneven cooking. Aim for 1.5 mm — thin enough to see through. Another pitfall is not drying the potato slices thoroughly; excess water can make the galette soggy.

Be patient with the onions; rushing them over high heat will burn them before they caramelize. Finally, don't skip the parchment paper — it ensures the galette releases cleanly from the skillet.

How to tell it’s done

The galette is done when the top potato slices are deeply golden and crisp at the edges, and a knife inserted into the center meets no resistance. The cheese should be melted and bubbling around the sides, and the bottom should be caramelized and sticky.

If you want extra crispness, you can place the skillet under the broiler for 1-2 minutes, watching closely. Let it rest for 5 minutes before unmolding to allow the layers to set.