Preheat the oven to 190°C (375°F).
In a medium saucepan, melt the butter over medium-high heat. Add the leeks and cook until they are tender. Set aside.
Peel and slice the potatoes into 1/8-inch (2mm) thick rounds using a mandoline or chef’s knife.
In an 8-inch (2L) baking dish, arrange a quarter of the potato slices in a slightly overlapping pattern. Season with salt and pepper. Sprinkle with a quarter of the cooked leeks, 1/4 cup (50ml) of Gruyère, and 1/4 cup (50ml) of heavy cream. Repeat for a total of four layers.
Pour the broth over the potatoes, cover with aluminum foil, and place the dish on a baking sheet to catch any liquid that may escape.
Cook for 1 hour and 20 minutes or until the potatoes are tender when pierced with the tip of a knife.
In a small bowl, mix the remaining Gruyère, panko, and walnuts. Season with salt and pepper.
Remove the dish from the oven, uncover, and evenly distribute the walnut mixture. Return to the oven for 15 minutes or until the walnuts are golden brown.