Potatoes Boulangère

• 3 sweet onions (such as Vidalia), sliced 
• 8 garlic cloves, thinly sliced 
• 1 teaspoon olive oil 
• Kosher salt 
• 8 thyme sprigs, stemmed, plus more for garnish 
• Freshly ground pepper 
• 4 cups chicken stock 
• 10 medium Yukon gold or other waxy potatoes, peeled 
• 1/2 cup (1 stick) cold butter, cubed

Preheat the oven to 350°F. 

In a large sauté pan over low heat, combine the onions, garlic, and oil and sprinkle with salt. Cook until translucent, about 10 minutes, being careful not to brown the onions. Add the thyme and adjust the seasoning with salt and pepper.

Warm the stock in a medium saucepan over low heat. Thinly slice the potatoes by hand or using a mandoline with a safety guard. In a 9 x 13-inch casserole, alternately layer the potatoes and cooked onions, overlapping the slices slightly in each layer. For each layer, season with salt and pepper, add a little warm stock to cover them, and dot with a few cubes of butter.

Bake, uncovered, for about 2 hours, or until the potatoes are fork tender. If the potatoes are browning too quickly, cover with parchment and aluminum foil, removing the parchment and foil for the last 15 minutes of cooking. This dish is best served warm but will reheat well. Garnish with more thyme leaves just before serving.