1. Place potatoes in a large saucepan and add enough water to cover; generously salt water and bring to a boil, adjusting the heat so that the water is bubbling, but not too rapidly. Cook until potatoes are tender, about 20 minutes.

2. Drain and mash or pass potatoes through a ricer into saucepan. Place over low heat and add milk; stir to combine. Add butter and stir until melted. Beat with a wooden spoon until stringy.

3. Remove from heat and stir in cheeses until melted; season with salt and pepper. Transfer to a bowl and drizzle with truffle oil. Serve with toasts.