Scalloped Hasselback Potatoes With Cheddar

Preheat oven to 450 degrees F. Line rimmed baking sheet with foil. Grease shallow 2-quart oval baking dish. In large bowl, combine Cheddar and pepper Jack; transfer one-third to small bowl and set aside. Add heavy cream, garlic, rosemary, 1 1 ⁄4 teaspoon salt, and 1 ⁄2 teaspoon pepper to large bowl with cheeses. Very thinly slice potatoes; place in bowl with cream mixture. Toss with hands until well coated. Arrange slices in baking dish, sides facing up, until entire dish is tightly packed (some slices may not fit). Pour remaining cream mixture over potatoes. Place baking dish on prepared baking sheet. Cover dish tightly with foil; bake 1 hour. Remove foil and sprinkle top with remaining cheese. Bake until potatoes are very tender and top is golden brown; 55 minutes to 1 hour 15 minutes. Let stand at least 15 minutes before serving.