Add 2 tbsp of unsalted butter and 2 tsp of olive oil into a medium sized sauce pan.
Place the pan over medium heat, and, once the butter is melted, toss in the onions.
Cook the onions for about 2 minutes, then add in the garlic.
Cook and stir for about 1 minute.
Now add in the cheese soup, heavy cream and milk. Stir until well combined.
Sprinkle in the black pepper, salt and nutmeg. Stir.
Add in the Gruyere cheese and stir.
Reduce the heat to low, and stir until all the cheese is melted.
Next, add the peeled & sliced potatoes into a well greased bake dish or cast iron skillet.
Pour the cheese sauce all over the potatoes.
Bake in the oven on 375° F for about 50 minutes, or until the potatoes are tender.
Remove from the oven, then sprinkle parsley flakes (optional) on top.
Let cool to a safe temperature.
Serve & enjoy!