Creamy Rosemary Au Gratin Potatoes

3 tablespoons butter, divided

3 tablespoons all-purpose flour

½ teaspoon sea salt

2 cups half-and-half

4 russet potatoes - scrubbed, unpeeled, and cut into 1/4-inch slices

1 onion, sliced into rings

1 tablespoon chopped fresh rosemary

½ cup shredded Cheddar cheese

sea salt and ground black pepper to taste

1 cup shredded Cheddar cheese

1 pinch paprika

Preheat oven to 400 degrees F (205 degrees C). Grease a 1-quart baking dish with about 1 teaspoon butter.

Melt remaining butter in a saucepan over medium heat. Add flour and 1/2 teaspoon sea salt; whisk constantly for 1 minute. Pour half-and-half into flour mixture, stirring and cooking until mixture has thickened, about 4 minutes.

Layer half the potatoes into the bottom of prepared baking dish. Top with onion slices, rosemary, 1/2 cup Cheddar cheese, and remaining potatoes; season with sea salt and pepper. Sprinkle 1 cup Cheddar cheese and paprika over potatoes. Pour half-and-half mixture over potato mixture; cover with aluminum foil.

Bake in preheated oven until potatoes are tender, about 75 minutes. Remove aluminum foil and continue baking until potatoes are lightly browned, about 15 minutes.