Preheat oven to 400º. Melt butter in a large, deep skillet or saucepan over medium high heat. Add leeks and garlic; sauté 5 to 7 minutes or until tender. Stir in rosemary, sea salt and pepper.
2
Add potatoes, heavy cream and light cream to leek mixture in pan, stirring gently to combine. Bring mixture to a boil, reduce heat and simmer, uncovered 10 to 15 minutes or until potatoes are barely tender (Do not overcook. Potatoes will be pliable, but not fully cooked.)
3
Spoon half of potato mixture into a lightly greased 2 1/2 quart baking dish. Sprinkle with 1/2 cup Gruyere cheese and 1/4 cup Parmesan. Top with remaining potato mixture; sprinkle with remaining cheeses. (May be covered and refrigerated overnight. Let come to room temperature before baking.)
4
Bake 20 to 30 minutes or until potatoes are tender and top is golden brown. Garnish, if desired. Makes 8 servings.
