Ingredients

• 2 tbsp butter or olive oil
• 2 large yellow onions, thinly sliced
• 1 tbsp balsamic vinegar (optional, deepens sweetness)
• 1½ lbs (700g) Yukon Gold or Russet potatoes, peeled & thinly sliced (⅛” thick)
• 1½ cups (360ml) heavy cream (or half-and-half for lighter version)
• 1 garlic clove, minced
• 1½ cups (150g) shredded Gruyère cheese
• ¼ tsp freshly grated nutmeg
• Salt & black pepper to taste
• Pinch of thyme leaves (optional)

Instructions

Caramelize onions: Melt 1 tbsp butter in a skillet over medium-low. Add onions; cook 25–30 min, stirring occasionally, until deep golden. Add balsamic vinegar (if using); cook 1 more min. Set aside.
Preheat oven to 375°F (190°C). Butter a 9×9″ or 2-qt baking dish.
In a bowl, whisk cream, garlic, nutmeg, salt, and pepper.
Layer:
– Bottom: ⅓ potatoes (slightly overlapping)
– ½ caramelized onions
– ½ Gruyère
– Drizzle with ⅓ cream mixture
Repeat layers. Top with remaining potatoes, remaining cream, and remaining cheese.
Cover with foil; bake 45 min. Uncover; bake 25–30 min more until top is golden and potatoes pierce easily with a knife.
Rest 15 min before serving—this sets the creamy layers beautifully.