4 large sweet potatoes (about 2 1/2 pounds)
2 tablespoons butter
1 large onion, sliced thin (or 2 medium)
4 garlic cloves, minced
1/4 cup fresh sage leaves, chopped (or 2 tablespoons dried sage)
1/4 teaspoon chipotle pepper powder (optional)
1 cup cream
1 1/2 cups milk
1 tablespoon butter
1/2 cup breadcrumbs
1/2 cup Parmesan
Heat the oven to 350°F and grease a 9x13 casserole or gratin dish. Peel and slice the potatoes. Toss them in a bowl with a little olive oil and salt and pepper.
Heat the butter in a heavy skillet and slowly caramelize the onions. When the onions are dark, add the garlic and cook just until golden. Reserve two tablespoons of the chopped sage (or 1 tablespoon if using dried) and add the rest to the onions, along with the chipotle powder, cream and milk. Cook until the cream mixture is slightly reduced then remove from the heat.
Layer the potatoes in a greased casserole dish with the onions, lifting them out of the cream with a slotted spoon. Pour the cream over the top and bake for about 30 minutes or until the potatoes are barely tender. If not enough liquid remains, add additional milk around the perimeter until there is 1/4-1/2 inch of liquid in the bottom of the dish.
Heat the remaining tablespoon of butter in the skillet and toast the reserved chopped sage and the breadcrumbs until golden and fragrant. Remove from heat and toss with the Parmesan. Sprinkle over top of the gratin and return to the oven for about 15 minutes or until brown and crispy.