Prudent Potatoes au Gratin
½ tablespoon unsalted butter
1 tablespoon olive oil
1 large yellow onion, trimmed, peeled and very thinly sliced (approximately 2 cups)
4 pounds russet potatoes
½ pound Gruyère cheese, grated
½ pound Comte or Fontina cheese, grated
½ cup freshly grated Parmesan cheese
1 tablespoon finely chopped fresh sage
1 teaspoon coarse sea salt or kosher salt
½ teaspoon freshly ground black pepper
2 cups heavy cream
Butter a 9-by-13-inch glass baking pan.
In a large sauté pan, melt the butter with the oil over medium-low heat, then add the onion. Cook over low heat, stirring frequently, until the onion is very limp and has caramelized without burning, 15 to 25 minutes. Be sure to cook until the onion has completely changed color.
Preheat the oven to 375°F.
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