Preheat the oven to 320°F. Begin by slicing the onion and placing it in a hot pan lightly greased with ghee. Cook the onions for up to 5 minutes while stirring over a medium heat until fragrant.
Add sliced bacon to the cooked onions. Then add thyme and cook until the bacon is lightly browned. Set aside.
While the bacon and onions rest, peel, then thinly slice the rutabaga into 1/4 cm slices. You should have 3 layers worth of rutabaga.
Grease a large baking dish with ghee and place one layer of sliced rutabaga. Cover the first layer with your bacon and onion mix with a half cup of the grated cheddar cheese. Repeat.
Finally, for the last layer of rutabaga slices, combine the cream, chicken stock, salt, and pepper in a separate bowl and pour over the last layer until almost covered. Season to your liking if needed.
Cover the baking dish with aluminum foil and place a cookie sheet or another baking dish underneath the gratin to catch any juice drippings. Bake for 1 hour, covered.
After the hour, the rutabaga will soften. Remove the aluminum foil and push the rutabaga down with a spatula to immerse it with the cream mixture (prevents drying out!). Bake for another 45-60 minutes while routinely pressing the rutabaga down every 15 minutes or until the rutabaga gets tender.
After the hour, take it out of the oven and spread the creme fraiche and grated parmesan cheese on top. Meanwhile, turn the oven up to 400°F.
Bake the gratin for another 20-30 minutes (until the cheese has melted).