Preheat oven to 375°F.

In a large oven-safe skillet, heat olive oil and butter over medium-low heat.

Add the thinly sliced onion and cook until very soft, about 15 minutes.

Stir in fresh rosemary, halved new potatoes, and peeled and sliced shallots.

Season with salt and pepper to taste.

Raise the heat to medium-high and cook until the potatoes begin to brown, about 10 minutes.

Remove from heat.

Add chicken stock to the skillet.

Cover the skillet and transfer to the preheated oven.

Cook until the potatoes are fork-tender, about 10 minutes.

Uncover the skillet and continue cooking until the liquid is reduced, about 20 minutes.

Serve hot.