Grease an oven dish with butter or nonstick spray.
Slice the sweet potatoes thinly with the peel on.
In a medium-size saucepan over medium-high heat, add the butter and melt.
Whisk in the flour and slowly whisk in the milk.
Add in the cheese and mix until cheese is melted. Set aside.
Layer the bottom of an oven dish with half of the sweet potatoes, sprinkle with thyme and seasoning. Layer the remaining sliced sweet potatoes, season and cover with the remaining cheese sauce.
(Add more milk to the sauce if it is too thick.) Bake in the oven for 25 to 30 minutes.