Begin by preheating your oven to 375°F (190°C) and lightly buttering a gratin dish with extra butter. In a frying pan, melt 6 tbsp butter over medium heat, then add 1 large onion, sliced, and cook gently while stirring occasionally for 3–5 minutes until the onions are softened but still pale in color.
This delicate cooking is essential—you want to develop the onion's natural sweetness without allowing them to brown or caramelize too much at this stage.
Once the onions are ready, begin layering the potatoes and onions in the prepared gratin dish. Use 2 lb (1 kg) of Russet potatoes, thickly sliced, and alternate layers of potatoes with the cooked onions, seasoning each layer generously with salt and black pepper.
Finish the dish with a neat, final layer of potatoes on top, which will create an attractive presentation as it bakes and browns slightly.
Pour any remaining butter left in the frying pan over the top of the layered potatoes, distributing it evenly across the surface.
Bake for 1–1½ hours until the potatoes are completely tender when pierced with a fork. The dish is ready when the top is lightly golden and the potatoes have absorbed the buttery, onion-infused flavors.
Remove from the oven and garnish with chopped parsley before serving hot as a classic French-inspired side dish.
