Peel the celeriac thickly (the skin is tough) and cut into even cubes, about 2–3 cm.

Place the celeriac cubes in a saucepan, cover with cold water, and add a pinch of salt. Bring to a boil, then reduce to a gentle simmer for 20–25 minutes, until very tender when pierced with a knife.

While the celeriac cooks, gently heat the milk, double cream, garlic, and rosemary in a small pan until just steaming. Do not let it boil. Remove from the heat and let it infuse while the celeriac finishes cooking.

Once the celeriac is tender, drain well and return it to the warm pan. Let it sit for a minute or two to steam off excess moisture — this prevents a watery mash.

Mash the celeriac until smooth (or use a hand blender for a purée).

Strain the infused milk mixture to remove the garlic and rosemary, then gradually stir it into the mash along with the butter and Dijon mustard. Adjust the texture with more milk if needed — it should be silky and spoonable. Taste and season with salt and black pepper.

Garnish with fresh herbs and/or toasted breadcrumbs.