Start by peeling and dicing your firm potatoes into bite-sized pieces. In a large pot, bring salted water to a boil and gently add the potatoes. Cook for about 15 minutes until they are slightly tender but still firm, stirring occasionally.

While the potatoes are cooking, heat a generous tablespoon of butter in a large skillet over medium heat. Add the sliced leeks and sauté until they soften, about 5 minutes. Then, introduce the torn savoy cabbage to the skillet and cook for another 10 minutes.

Once the cabbage is tender, pour in a splash of white balsamic vinegar and add salt, pepper, sugar, and a pinch of freshly grated nutmeg. Mix thoroughly and allow to simmer for an additional 15 minutes.

Preheat your oven to 200°C (around 400°F). Grease a large baking dish with a bit of butter.

Drain the partially cooked potatoes and layer them evenly in the bottom of the greased baking dish. Then, add the sautéed leeks and savoy cabbage mixture on top.

Grate a generous amount of Gouda cheese and sprinkle it liberally over the layered vegetables in the baking dish.

Place the casserole in the preheated oven and bake for 20 minutes, or until the top is bubbling and golden-brown.