Step 1

Preheat the oven to 350 degrees.

Step 2

Remove the skins from the potatoes and chop them roughly. Add the onions, salt, and pepper.

Step 3

Add the butter and oil to a cast iron (or other oven safe) skillet over medium high heat. When the butter has melted, add the potato mixture. Press the mixture firmly into the pan.

Step 4

Cook for 5-6 minutes or until the potatoes are golden brown on the bottom. Flip the potatoes and cook for an additional 2-3 minutes.

Step 5

Remove the skillet from the heat and spread with the sour cream. Top with the grated Gruyere cheese.

Step 6

Place the skillet in the oven and bake until the cheese is melted and bubbly. Allow to stand for a few minutes. Cut into wedges and serve.