Start by placing the thinly sliced potatoes and pumpkin by layers in a gratin dish and set aside.
For the bechamel, use a medium saucepan to heat the butter over medium-low heat until melted. Add the corn starch and stir until smooth. Cook until the mixture turns light golden, about 5 minutes.
Meanwhile, heat the milk and add it to the butter mixture in three times, whisking continuously until very smooth. Bring to a boil. Cook 10 minutes, stirring constantly, then remove from heat. Season with salt, nutmeg and pepper, and pour it over the potatoes and pumpkin.
Preheat the oven to 350ºF.
Transfer gratin dish to oven, and bake for 45 minutes to one hour.