Start by cutting the top and bottom off the celeriac. Use a sharp knife to carefully slice away the rough, hairy skin. Cut the white flesh into small, even cubes for quick cooking.

Peel your potato and cut it into cubes of the same size as the celeriac. Adding a little potato helps give the puree a better, more stable texture.

Place the celeriac and potato cubes into a large saucepan. Cover them with the milk and a pinch of salt. Bring the milk to a gentle simmer over medium heat.

Lower the heat and cover the pan with a lid. Let the vegetables simmer for about 15 to 20 minutes. They should be very soft when pierced with a fork.

Drain the vegetables through a colander but keep a little of the warm milk. This leftover milk is useful if you need to thin out the puree later.

Put the cooked vegetables into a food processor while they are still hot. Add the butter, nutmeg, and the optional cream. Blend until the mixture is completely smooth and silky.

Taste your puree and add salt, pepper and a tiny squeeze of lemon. The lemon juice helps brighten the earthy flavor. Pulse one last time to mix everything perfectly.