- 3 Tbsp unsalted butter, divided
- 1 onion, halved and thinly sliced into half moons
- Salt and freshly ground white pepper
- 2 lb sweet potatoes
- 1 lb Yukon gold potatoes
- 4 cloves garlic
- 1/4 cup fresh sage leaves, finely chopped
- 1 cup heavy cream
- 1 Tbsp unsalted butter
- 2 Tbsp finely chopped sage leaves
- 1/2 cup dried breadcrumbs or panko
- 1/2 cup grated Gruyere cheese
- Olive oil
Preheat oven to 350° and lightly butter an oval casserole dish with 1 tablespoon butter.
In a large frying pan over medium heat, melt remaining 2 tablespoons butter. Add onions, sprinkle with salt and cook, stirring occasionally, until light brown, about 15 minutes.
Meanwhile, cut potatoes into 1/8” thick slices and place in a bowl of cool water. Stand each slice of potato upright, alternating white and orange potatoes to make a circle. Repeat until all potatoes are used. Season with salt and pepper.
When onions are caramelized, add garlic and cook until fragrant, about 1 minute. Add sage, and cream. Bring to a simmer and cook until cream is slightly reduced, about 5 minutes, then remove from heat.
Spoon onions over potatoes. Season with salt and pepper. Pour cream over top.
Bake until potatoes are just tender and cream is golden brown and bubbling, 30-45 minutes.
Meanwhile, in a frying pan over medium-high heat, melt 1 tablespoon butter. Add sage and breadcrumbs and cook until golden and fragrant, about 3 minutes. Remove from heat and toss with cheese. Sprinkle mixture on top of gratin and drizzle top lightly with olive oil. Return to dish to oven until brown and crispy, about 15 minutes.