Place a frying pan on medium/high heat and add a small dash of sunflower oil.
Add the celeriac and fry briefly on both sides.
Reduce the heat slightly and add the butter, rosemary and garlic. (NB(if the pan is too hot the butter will burn!)
Brown the celeriac with the butter (This means that you pour some of the warm golden butter over the celeriac with a spoon, so that the celeriac gets a nice color and the flavors of the rosemary and garlic)
In the meantime, season the celeriac with salt and pepper.
Fry the celeriac until you can easily pierce it with a skewer and it has a nice golden brown color.
Toast the inside of the brioche buns.