What I can say? It's the power of sweet potatoes in cream.
This dish is not for dieters; it would be a dessert too if not for the sage and chipotle that give it fullness and a kick. Unless you cannot tolerate any heat at all, do use the chipotle; it adds a warmth that balances the richness of sweet potatoes, layered with cream and caramelized onions under a crunchy Parmesan crust.
Sweet Potato and Sage Gratin
2 tablespoons unsalted butter
2 medium onions, peeled and sliced into thin half moons
3 pounds sweet potatoes or yams (3 to 4 large potatoes)
Kosher salt and freshly ground black pepper
4 garlic cloves, minced
1/4 cup fresh sage leaves, finely chopped
1/4 teaspoon chipotle pepper powder (optional)
1 cup heavy cream
1 tablespoon unsalted butter
2 tablespoons finely chopped sage leaves
1/2 cup fine dried breadcrumbs
1/2 cup grated Parmesan
Heat the oven to 350°F and lightly grease a 9x13 casserole or gratin dish with olive oil or butter. Heat the butter in a heavy skillet (cast iron, ideally) and when it foams up, add the onions. Sprinkle them lightly with salt. Slowly caramelize the onions over medium heat, stirring frequently. This will take at least 15 minutes.
While the onions are caramelizing, peel and slice the potatoes in thicknesses of about 1/4-inch. Tile half of them in a the prepared casserole dish, overlapping them in tight rows or a spiral. Sprinkle them lightly with salt and pepper.
When the onions are dark brown, add the garlic and cook just until golden. Finely chop the sage and add this to the onions, along with the chipotle powder and cream. Bring to a simmer and cook until the cream is slightly reduced then remove from the heat.
Use a slotted spoon to remove the onions and garlic from the cream. Spread the onions over the layer of sweet potatoes in the dish. Layer the remaining potatoes on top, forming a tight spiral or rows. Sprinkle lightly with kosher salt and black pepper. Pour the cream over the top. It will look like there isn't enough cream; it won't cover the potatoes or fill up the dish. Resist the impulse to add more cream, however; there is plenty to cook the potatoes, and adding more may cause the potatoes to get mushy and fall apart.
Bake uncovered for 25 to 30 minutes or until the potatoes are just tender and the cream is golden brown and bubbling. (Note: Baking time is highly dependent on how thick you cut your potatoes. The 25 to 30 minute time is based on 1/4-inch thick slices. Any thicker and the baking time may rise.)
Meanwhile, heat the remaining tablespoon of butter in the skillet and toast the reserved chopped sage and the breadcrumbs until golden and fragrant. Remove from the heat and toss with the Parmesan. Sprinkle this over top of the gratin and drizzle the top lightly with olive oil. Return to the dish to the oven for about 15 minutes or until brown and crispy. Let cool for at least 15 minutes before slicing and serving.
• Gruyere is another cheese that is excellent in the breadcrumb topping.
• This recipe makes a full 9x13-inch dish, but there are only two layers of potatoes. If you would like to maker a deeper gratin and increase the amount of servings, double the onions and increase the potatoes by 50%. Then create a second layer of caramelized onions and sweet potatoes on top of the first. Increase the cream by 50% as well, and expect to bake at least 10 minutes longer.