This rich gratin uses a combination of milk and half-and-half instead of the traditional heavy cream to slash saturated fat. We find the texture of Kerrygold reduced-fat cheese more like full-fat cheese than other brands.
Preheat oven to 400°.
Coat an 11 x 7-inch glass or ceramic 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 oil; swirl to coat. Add onion and garlic; cook 5 minutes, stirring occasionally. Weigh or lightly spoon flour into a dry measuring cup. Combine flour, milk, half-and-half, salt, nutmeg, and pepper in a medium bowl, stirring well with a whisk. Add milk mixture to pan; cook 3 minutes or until smooth and mixture thickens slightly, stirring constantly with a whisk. 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.