Baked Shells with Gremolata Breadcrumbs

Preheat oven to 350°F with rack 10 inches from heat. Bring a large saucepan of water to a boil over high. Add 1 tablespoon salt, and stir in shells; cook according to package directions for al dente. Drain shells, and transfer to a large bowl. Add 1 tablespoon oil, and toss to coat. Set cooked shells aside. 

Wipe saucepan clean. Add butter, and melt over medium. Whisk in flour; cook, whisking constantly, until mixture is lightly browned, 3 to 4 minutes. Gradually whisk in half-and-half and milk until sauce is smooth. Bring just to a simmer over medium-high, whisking occasionally. Reduce heat to low; simmer, whisking often, until no floury taste remains, about 5 minutes. Remove from heat. Gradually whisk in 1 1/2 cups fontina and 1 1/2 cups cheddar until smooth. Whisk in onion powder, garlic powder, white pepper, and nutmeg. Fold in cooked shells. Season with 1 1/2 teaspoons salt and black pepper. Spread mixture evenly in a 2-quart broiler-safe baking dish. Sprinkle with remaining 1/2 cup fontina and remaining 1/2 cup cheddar. 

Bake in preheated oven until bubbling around edges, about 20 minutes. Increase oven temperature to broil. (Do not remove baking dish from oven.) Broil until browned in spots, 3 to 4 minutes. Remove from oven; let cool 5 minutes. 

Meanwhile, heat remaining 2 tablespoons oil in a medium skillet over medium. Add panko; cook, stirring constantly, until deep golden, about 4 minutes. Transfer to a medium bowl, and let cool 2 minutes. Stir in parsley, lemon peel, and garlic. Season with remaining 1/4 teaspoon salt. Serve gremolata breadcrumbs with shells.