Preheat your oven to 350°F (175°C).

Grease a medium-sized baking dish with a bit of the butter to prevent sticking.

Melt 2 tablespoons of butter in a skillet over medium heat.

Add the thinly sliced onions and cook for 8-10 minutes, stirring occasionally, until they are softened and slightly golden.

While the onions are cooking, wash and thinly slice the potatoes (about 1/8 inch thick) using a sharp knife or mandoline for even slices.

In the greased baking dish, layer half of the sliced potatoes evenly across the bottom.

Season with 1 teaspoon of salt and ½ teaspoon of black pepper.

Spread half of the sautéed onions over the potatoes, followed by half of the chopped rosemary.

Repeat the layers with the remaining potatoes, salt, pepper, onions, and rosemary.

Pour the vegetable broth or stock evenly over the layered potatoes and onions.

Dot the top of the dish with the remaining 1 tablespoon of butter.

Cover the dish with aluminum foil and bake in the preheated oven for 45 minutes.

After 45 minutes, remove the foil and bake for an additional 25-30 minutes, or until the top is golden and crispy and the potatoes are tender when pierced with a fork.

Let the dish rest for a few minutes before serving.

Serve as a side dish with roast meats, chicken, or on its own for a hearty vegetarian option.