Peel the potatoes, put them in salted boiling water and cook until soft.
Clean the cauliflower, remove the leaves and steam, cut into large florets.
Boil the egg hard and rinse it in cold water for easier peeling.
Melt butter in a saucepan over low heat and stir in flour until smooth. Add the milk, stirring constantly, so that nothing burns. Remove the saucepan from the hob, add the mustard and stir well.
Season the mustard sauce with salt, pepper and parsley