Set a steamer basket in a large pot. Fill with water to just below the basket and bring to a boil. Add the potatoes to the basket and reduce the heat to a simmer. Cover and cook until the potatoes are tender, about 15 minutes. Remove from the steamer basket and allow to cool slightly.
Meanwhile, combine the fontina, Emmemtal and Gruyere in a medium bowl and toss with the cornstarch until evenly coated. Scatter in a 12-inch cast-iron skillet. Bake until the cheeses are completely melted, about 15 minutes, then whisk briefly until smooth. Serve immediately with the steamed potatoes, sausages, baguette slices and cornichons.