Step 2

Preheat oven to 350F°.

Step 3

Melt 2 tablespoons butter in a skillet over medium-high heat. Add onions and sauté for 5 minutes.

Step 4

Add garlic and one more tablespoon of butter. Sauté for another 5 minutes.

Step 5

Peel potatoes and slice thin using a food processor or knife (see note)

Step 6

Butter an 11" oval baking dish and start assembling the casserole. Arrange 1/3 of the potatoes in a single layer. Top with 1/2 the onions, 1/3 of the rosemary, salt, and pepper (season every layer).

Step 7

Repeat with another layer of potatoes, then onions, rosemary, salt and pepper.

Step 8

Arrange the top layer of potatoes so that the rounds overlap each other slightly.

Step 9

Add stock so it submerges most of the potatoes, stopping before it covers the top layer (so that it will get crispy).

Step 10

Top with remaining rosemary, salt, and pepper.

Step 11

Add little pieces of butter (1/3" squares) to top of potatoes (about 2 tablespoons) and drizzle with extra virgin olive oil.

Step 12

Cover with foil and bake for one hour.

Step 13

Remove the foil, raise the oven temperature to 425°F and bake for another half hour.

Step 14

Shred ½ cup Gruyere cheese and chop the parsley.

Step 15

Remove the potatoes from the oven, sprinkle with cheese and return to the oven for 5 minutes, until the cheese has melted.

Step 16

Serve with chopped parsley and enjoy!