Butter six ¾-cup (6–fl oz/180-ml) ramekins or other individual baking dishes. Place the dishes on a rimmed baking sheet.
Cut the leek in half lengthwise, then thinly slice crosswise. Put the slices in a bowl generously filled with cold water and swish with your hand to release any grit. Set the bowl next to the stove. In a large sauté pan, melt the 3 tbsp butter over medium-high heat. Add the onions and 1 tsp salt and sauté until soft, about 4 minutes. Using a slotted spoon, transfer the leeks from the water to the pan, then reduce the heat to medium and cook, stirring often, until the onions and leek are very soft, about 12 minutes. Pour in the wine and cook for 1 minute. Remove from the heat.
Preheat the oven to 375°F (190°C). Place the bread cubes in a large bowl and add the onion-leek mixture, cream, broth, ⅔ cup (2½ oz/ 80 g) of the cheese, the nutmeg, and a few grinds of pepper. Stir well, then let stand for 10 minutes.
Divide the bread mixture evenly among the prepared ramekins. Sprinkle the with the remaining ⅓ cup (1½ oz/45 g) cheese. Bake until the tops are crusty and the panades are warmed through, about 30 minutes. Let cool for about 5 minutes before serving.