Hearty Swiss Potato Casserole

Melt butter in a saucepan over medium-low heat. Let simmer for 1 minute. Add flour 1 tablespoon at a time whisking constantly until completely incorporated 5 to 8 minutes. Add 1 cup water slowly and whisking constantly to avoid clumping. Remove from heat.

Whisk remaining 2/3 cup water together with cornstarch in a small bowl. Add slurry to the gravy; whisk to combine.

Preheat the oven to 375 degrees F (190 degrees C).

Stir the gravy briefly; season with salt and pepper. Add thyme and rosemary. Stir in Swiss cheese and Gruyere cheese. Add potatoes scallions and garlic; stir well until potato slices are separated and coated with gravy. Pour mixture into an 8-inch square baking dish.

Bake in the preheated oven until potatoes are tender 40 to 50 minutes.