Creamy, comforting, and layered with flavor, this potato and Swiss cheese gratin is a cozy crowd-pleaser perfect for any occasion. Thinly sliced russet potatoes and sweet onions are coated in a smooth, herb-infused cheese sauce made with lactose-free milk and melty Swiss cheese. Baked until tender and topped with a golden, bubbly layer of cheese, this gratin is hearty enough for a main dish or the perfect side to elevate any meal. You can even prepare it a day ahead and simply reheat before serving, and for a fun twist, try swapping in different types of cheese to complement the creamy sauce and bring out new flavor combinations.
Preheat oven to 350°F. Coat a 9-inch pie plate or a small casserole dish with cooking spray.
In a small sauce pan, melt butter over medium heat; add flour, whisking for 2 to 3 minutes or until incorporated.
Gradually add milk, stirring until smooth.
Add salt, pepper, garlic powder, nutmeg, and thyme; bring to a boil over medium-high heat, then lower to medium-low heat. Simmer for 5 to 7 minutes or until sauce is thickened.
Add 1-1/2 cups of shredded Swiss cheese, stirring until cheese is melted; cook for an additional 3 to 5 minutes over medium-low heat until sauce is smooth.
In a large bowl, combine potatoes and onions; add cheese sauce and toss to coat.
Transfer potato mixture to prepared pie plate; wrap tightly with foil.
Bake for 45 to 50 minutes or until potatoes are tender; remove foil.
Top with remaining 1/2 cup of shredded Swiss cheese; bake, uncovered, for an additional 5 to 10 minutes or until cheese is melted and lightly browned.
