Boil the potatoes in a large pot of salted water until they are tender (about 15-20 minutes). Drain the potatoes and return them to the pot. Add the plant-based milk and vegan butter. Mash the potatoes until smooth and creamy. Season with salt, pepper and nutmeg, then set aside to cool down.
Preheat the oven to 200°C (390°F).
In a mixing bowl, combine the mashed potato with vegan mozzarella, vegan Parmesan and diced vegan.
Grease a baking dish with a bit of olive oil and sprinkle around half of the breadcrumbs.
Spread the mashed potato mix in an even layer and level it with a spatula.
Sprinkle with the remaining breadcrumbs, and drizzle with olive oil.
Bake in the preheated oven for 30-40 minutes, until the top is golden and crispy.
Let the gateau cool for a few minutes before serving.