Preheat the oven to 450 degrees.

Coat an oven safe casserole dish with non-stick spray.

Arrange a layer of thinly sliced sweet potatoes, slightly overlapping in the dish.

Sprinkle about 1 tablespoon of flour evenly over the sweet potatoes.

Season the layer with salt and pepper.

Sprinkle some tarragon over the sweet potatoes.

Dot 1 tablespoon of butter (cut into small pieces) over the layer.

Sprinkle 1 tablespoon of shredded cheddar cheese over the layer.

Repeat the layering process with thinly sliced onions.

Repeat the layering process with thinly sliced celeriac.

Top each onion and celeriac layer with flour, butter, salt, pepper, tarragon, and cheese.

Complete the gratin with a final layer of sweet potatoes.

Pour 1 cup of heavy cream slowly over the top of the casserole, allowing it to sink into the layers.

Season the top of the casserole with salt and pepper.

Cover the casserole dish tightly with tinfoil.

Bake in the preheated oven for about 45 minutes, or until the vegetables are tender.

Remove the tinfoil from the casserole.

Cover the gratin evenly with the remaining shredded cheddar cheese.

Return the uncovered gratin to the oven for an additional 5 - 10 minutes, or until the cheese is melted and bubbly.