Prep the beets and potatoes by slicing off the end and using a peeler to remove the remaining skin.
Using a mandolin on the thinnest slice setting, slice the potatoes, beets and onion. Keep them separate for now.
Use a small amount of butter to grease the bottom of a 10 inch cast iron skillet.
Create a layer of beet slices on the bottom of the skillet using half of the beet slices, then scatter ⅓ of the onions on top. Sprinkle with rosemary and thyme and a few dollops of butter then season with salt and pepper.
Repeat with the remaining beet slices.
Create a layer of potato slices on top of the beet layer. Scatter the remaining onions, rosemary and thyme on top then spread the remaining butter across the layer.
Top with a layer of the remaining potato.
Pour the stock evenly over the dish, then drizzle the top with a bit of olive oil.
Bake the skillet in the oven for 25 minutes, then reduce the heat to 350℉ and bake for another 30 mins.
Remove skillet from the oven and allow it to cool about 10 minutes before serving.