Boil the potatoes until tender.

Mash them well, season with salt, pepper, and a splash of olive oil. Spread a layer of mashed potatoes in a baking dish.

Dice the bacon and mushrooms.

Wash the spinach thoroughly, dry it, and chop it into small pieces (including the stems if you like, as they will shrink when cooked).

In a skillet, cook the bacon with a little oil (the bacon will release its own fat). Remove the bacon and, in the same skillet, add olive oil and butter. Once hot, add the mushrooms and cook for a few minutes, then add the spinach. Stir, add the bacon back in, season with salt, stir again, and remove from heat.

Pour this mixture over the layer of potatoes in the baking dish, spread evenly, and cover with another layer of mashed potatoes.

Use a fork to make lines across the top, sprinkle with Parmesan cheese (or another cheese if you prefer), and dot with small cubes of butter. Bake at 375°F (190°C) for 25 minutes.