- 2 Tbsp unsalted butter
- 2 yellow onions, halved and thinly sliced into half moons
- 2 lbs sweet potatoes
- 1 lb Yukon gold potatoes
- Freshly ground pepper
- 4 cloves garlic, minced
- 1/4 cup fresh sage leaves, finely chopped
- 1/4 tsp cayenne, (optional)
- 1 cup heavy cream
- 1 Tbsp uinsalted butter
- 2 Tbsp finely chopped sage leaves
- 1/2 cup fine dried breadcrumbs
- 1/2 cup grated parmesan or gruyere
- Olive oil
Preheat oven to 350° and lightly grease a 9″ by 13″ baking dish.
In a large frying pan over medium heat, melt butter. Add onions, sprinkle them lightly with salt and cook, stirring occasionally, until light brown, about 15 minutes.
Meanwhile, cut potatoes into 1/4″-thick slices. Shingle half of each potato in prepared dish, overlapping them in tight rows or a spiral. Season with salt and pepper.
When onions are caramelized, add garlic and cook until fragrant, about 1 minute. Add sage, cayenne, if using, and cream. Bring to a simmer and cook until cream is slightly reduced then remove from heat.
Spoon onions over potatoes. Layer remaining potatoes on top, forming a tight spiral or rows. Season with salt and pepper. Pour cream over top.
Bake until potatoes are just tender and cream is golden brown and bubbling, 25-30 minutes.
Meanwhile, in a small 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 for about 15 minutes or until brown and crispy. Let cool for at least 15 minutes before slicing.
Adapted from Thekitchn.com