Thinly slice the parsnips into rounds and the onion into rounds or half rounds. We like to use a Japanese mandoline to get uniform, thin slices.
Cook the onion in a skillet on medium with a bit of butter until softened or slightly browned and caramelized.
Remove, allow to cool and in the same skillet, build layers of overlapping parsnip rounds in a concentric circle brushed with melted butter, caramelized onions, cheese, and fresh leaves of thyme or another fresh herb that you prefer.
Finish the top with a layer of cheese, thyme, salt, and pepper. Bake at 400F for 30 minutes or until golden brown.
The Taste Edit - Parsnip Gratain https://thetasteedit.com/parsnip-gratin/