Heat oven to 375°F. Cook shells with tablespoon salt to 2 minutes short of package time. Gently drain; separate on plate.
Mix ricotta, egg, 2 oz mozzarella, 1/4 cup Parmesan, 1/4 teaspoon salt, pepper, and nutmeg. Melt butter in saucepan; whisk flour 1 minute.
Gradually whisk in milk and cream; simmer 3 minutes. Remove from heat; add remaining Parmesan and salt. Spread 1/2 cup sauce in 9-by-13-inch dish.
Evenly fill shells; arrange open-side up. Spoon remaining sauce over and between; top with remaining mozzarella.
Cover; bake 25 minutes, then uncovered 5 to 8 minutes until bubbling and centers 165°F. Rest 5 minutes; serve three shells per person.