Preheat oven to 190°C (375°F). Lightly grease a 9x13 inch baking dish on the bottom and sides.

Peel and slice potatoes to approximately 3mm thickness. Soak in cold water, then drain and pat each slice thoroughly dry with a clean towel.

Melt butter in a large saucepan over medium heat. Cook diced onion for 5 to 6 minutes until soft and translucent. Add garlic and stir for 60 seconds until fragrant. Pour in heavy cream and vegetable broth. Add salt, pepper, smoked paprika, and nutmeg. Bring to a gentle simmer over medium-high heat, stirring. Remove from heat.

Layer one-third of the dried potato slices in the baking dish, slightly overlapping. Pour one-third of the warm cream sauce evenly over the top. Scatter 1/2 cup of cheddar over the sauce. Repeat this sequence two more times with the remaining potatoes, sauce, and cheddar. Pour the final third of sauce over the top layer and scatter the remaining 1/2 cup cheddar plus all the Gruyère across the surface.

Cover the dish tightly with aluminum foil, sealing all edges. Bake at 190°C (375°F) for 35 minutes. Test potato doneness at the 35-minute mark with a thin knife; if there’s resistance, continue covered for another 5 to 8 minutes.

Remove foil. Increase oven temperature to 220°C (425°F). Bake uncovered for 15 to 20 minutes until the cheese surface is deeply golden with darker patches at the edges and the sauce is bubbling actively.

Rest uncovered for 10 minutes before serving. Scatter fresh chives or parsley over the top and bring the dish directly to the table.