Preheat oven to 400°. 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.
Add potatoes, cream, and milk 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.)
Spoon half of potato mixture into a lightly greased 2 1/2-quart baking dish. Sprinkle with 1/2 cup Gruyère 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.)
Bake 20 to 30 minutes or until potatoes are tender and top is golden brown. Garnish, if desired.
