Preheat oven to 350 degrees Fahrenheit.

Use a mandolin slicer to thinly slice the potatoes.

Spray an 11x7 inch pan with cooking spray to prevent sticking.

Layer the sliced potatoes evenly in the prepared pan.

To make the white sauce, melt 4 tablespoons of butter in a saucepan over medium heat.

Whisk in the flour, salt, and pepper to form a roux.

Remove the saucepan from the heat and gradually whisk in the milk until smooth.

Return the saucepan to medium heat and stir constantly until the sauce comes to a boil and thickens, about 1 minute.

Reduce heat to low. Add the grated Gruyere and smoked cheddar cheeses and stir until melted and smooth.

Pour the cheese sauce evenly over the layered potatoes in the pan.

Cover the pan with foil or a lid.

Bake in the preheated oven for 1 hour and 15 minutes, or until the potatoes are tender.

While the potatoes are baking, melt the remaining 4 tablespoons of butter in a skillet over medium heat.

Add the panko breadcrumbs and French's French fried onions to the melted butter and toss to combine.

Once the potatoes are tender, remove the foil or lid from the pan.

Spread the panko and onion mixture evenly over the top of the potatoes.

Turn on the broiler and broil until the panko topping is golden brown and crispy, watching carefully to prevent burning.

Remove from oven and let stand for a few minutes before serving.