Prepare the PotatoesPeel the potatoes and slice them evenly, about ⅛ inch thick, so they cook uniformly. You can use a mandoline or a sharp knife for precision. Rinse them briefly in cold water to remove excess starch.

Cook the Base FlavorHeat olive oil and butter in a large skillet over medium heat. Add the finely chopped onion and sauté for about 3–4 minutes until translucent. Add the minced garlic and cook for another minute, stirring frequently to prevent burning. The aroma at this stage should be rich and buttery.

Make the Creamy SauceSprinkle the flour into the pan and stir constantly to form a roux. Cook for about one minute to remove the raw flour taste. Slowly pour in the milk while whisking continuously until a smooth sauce forms. Add salt, pepper, marjoram, and a pinch of nutmeg. Stir until the sauce thickens slightly and becomes velvety.

Add the PotatoesGently add the sliced potatoes into the sauce, stirring carefully to coat each slice with the creamy mixture. Cover and cook over low heat for about 20 minutes, or until the potatoes begin to soften but are not fully cooked.

Incorporate the SpinachAdd the spinach directly into the pan with the potatoes. Stir gently, then cover again and cook for another 5 minutes. The spinach will wilt and blend beautifully into the creamy sauce.

Prepare for BakingPreheat your oven to 190°C (375°F). Grease a baking dish with a bit of butter or oil. Pour the potato and spinach mixture into the dish, spreading it evenly.

Top with Cheese and BakeSprinkle the grated cheese over the top. Bake in the preheated oven for 20–25 minutes, or until the top is golden and bubbling. The cheese should melt into a gooey, delicious layer of comfort.

ServeRemove from the oven and let the dish rest for about 5–10 minutes before serving. This allows the creamy sauce to settle, making it easier to cut and serve. Serve warm and enjoy the creamy, cheesy goodness with every bite.