Coat an oven safe casseraole dish with non-stick spray.
Arrange a layer of sweet potatoes, slightly overlapping. Sprinkle about a TB of flour on the potatoes, followed by salt and pepper, tarragon, 1 tb of butter (cut into small pieces)and 1 TB of cheddar.
Repeat this process with the onions and celeriac, topping each layer with flour, butter, seasonings and cheese.
Complete the gratin with a layer of sweet potatoes. Pour the cup of cream over the top of the casserole (slowly so it doesn't overflow). When it has sunk into the layers, season the top of the casserole, cover with tinfoil and bake until the vegetables are tender (about 45 minutes).
Cover the gratin with the remaining cheddar cheese and return to the oven uncovered for an additonal 5 - 10 minutes so the cheese can melt.