• Preheat oven to 400°.
• Coat an 11 x 7-inch glass baking dish with cooking spray.
• Place potatoes in a large saucepan; fill pan with enough cold water to cover potatoes by 2 inches. Place pan over high heat. Bring to a boil; reduce heat, and simmer 10 minutes or just until tender. Drain well.
• Gently layer one-third of potatoes in overlapping rows in prepared dish.
• Heat a medium saucepan over medium heat. Add the oil, swirling to coat.
• Add the onion and garlic; cook 5 minutes, stirring occasionally.
• Combine the flour, milk, half-and-half, salt, nutmeg, and pepper in a medium bowl, stirring well with a whisk.
• Add the milk mixture to pan; cook 3 minutes or until smooth and mixture thickens slightly, stirring constantly with a whisk.
• Add the sherry to the mixture and stir well to combine.
• Remove from heat; let stand 10 minutes. Strain through a fine sieve into a bowl; discard solids.
• Pour one-third of milk mixture over potatoes in baking dish.
• Repeat layers twice with remaining potatoes and milk mixture. Sprinkle evenly with cheese.
• Place pan in upper third of oven; bake at 400° for 12 minutes or until bubbling around edges. Turn broiler to high, leaving pan in oven; broil 1 minute or until browned.
• Let stand 10 minutes before serving.

Serves 10