Peel celeriac and cut into 2cm cubes. Place in a saucepan then just cover with water. Add half the lemon juice, salt and sugar.
Cover with a lid and simmer for 15-20 minutes until the celeriac is cooked through.
Remove the celeriac from the saucepan and place in a serving bowl. Retain the liquid.
Mix the remaining lemon juice with the egg yolks. Mix in 2-3 tablespoons of the cooking water and pour into a small saucepan.
Heat at low heat stirring continuously but do not allow to boil otherwise the egg will curdle. Season to taste. If the sauce is too sour stir in a bit more of the celeriac liquid. Pour sauce over the celeriac cubes and serve.