Place peeled chestnuts in a pot, add enough water to cover them, and boil for about 10 minutes until tender but not falling apart.

Drain the boiled chestnuts using a colander and let them cool.

In a pan, melt butter and sauté the sliced onion until it becomes translucent, then remove from heat.

Reserve a few chestnuts for garnish and mash them for decoration. Optionally, prepare a sprig of herbs for garnish.

In a blender, combine the drained chestnuts, sautéed onion, and 3 cups of milk. Blend until smooth.

Transfer the blended mixture to a pot, add the remaining 1.5 cups of milk, and heat while stirring. Once it starts to boil, season with salt to taste.