Yield: 12 servings.
4 pounds baking potatoes, washed, peeled, and halved
2 tablespoons unsalted butter, plus more for baking dish
1 onion, finely chopped
2 cloves garlic, finely chopped
1 teaspoon Dijon mustard
¼ teaspoon ground nutmeg
1 cup shredded Gruyère (about 5½ ounces)
1 cup shredded extra-sharp Cheddar
8 ounces cream cheese, softened
2 teaspoons salt
1 teaspoon black pepper
2 cups heavy cream
1 bay leaf
¾ cup grated Parmesan
For the fried onion topping:
1 onion, sliced very thin
2 cups buttermilk
2 cups all-purpose flour
1 tablespoon salt
Black pepper to taste
1 to 2 quarts canola oil
Bring a large pot of salted water to a boil. Add potatoes to boiling water, reduce heat to medium, and cook for 15 to 25 minutes or until they are not quite tender. Drain and set aside until potatoes are cool enough to handle.
In a small skillet, melt 2 tablespoons butter over medium heat. Add chopped onion and cook, stirring occasionally, until tender, about 6 minutes. Add garlic, mustard, and nutmeg, stirring about 2 minutes longer, until fragrant. Transfer to a medium bowl and let cool.
Preheat oven to 350°. Generously butter a 9 x 13-inch baking dish and place it on a baking sheet.
Add Gruyère, Cheddar, cream cheese, salt, and pepper to onion mixture and stir until well combined.
Slice potatoes ¼-inch thick. Arrange a third of the potatoes in baking dish, overlapping so there are no gaps, and spread with a third of the cheese mixture. Repeat layering twice, finishing with cheese. Pour heavy cream over potatoes and place bay leaf on top. Bake until cream is mostly absorbed and top is golden brown and bubbling, about 1 hour. Remove bay leaf. Sprinkle top with Parmesan cheese, and bake for an additional 15 minutes or until Parmesan is golden brown. Let cool for 10 minutes before serving. Top with fried onions.
For the fried onion topping: Place sliced onions in a baking dish and cover with buttermilk. Soak for at least an hour. Combine dry ingredients and set aside.
Heat oil to 375°. Place a handful of onions in flour mixture to coat. Place coated onions in a frying basket, or use tongs, and tap to shake off excess flour. Place onions into hot oil. Fry for a few minutes and remove as soon as golden brown. Place fried onions on a wire cooling rack over paper towels to drain. Repeat until all onions are fried.