Peel, wash and quarter the potatoes and cook in boiling salted water for about 20 minutes. In the meantime peel celery and cut it into slices of about 5 mm thickness. Boil up salted water with lemon juice and cook the celery slices in it for 6-8 minutes.
Remove and put aside. Drain the beetroot. Peel 1 onion and chop finely. Season vinegar with salt and pepper and stir in 2 tablespoons of oil bit by bit. Sprinkle beetroot with the vinaigrette, mix with the onion and set aside.
Peel the remaining onions and cut them into rings. Heat the remaining oil in a frying pan and fry the onion rings for 2-3 minutes until golden brown. Remove, degrease on kitchen paper and put aside.
Pass frying fat through a sieve. Whisk the eggs. Dab celery slices dry and season with salt and pepper. Fill 2 celery slices with 1 slice of ham and 1 slice of cheese. First pass through the flour, then the egg and turn into breadcrumbs.
Bread each celery cordon bleu twice. Drain the potatoes and let them evaporate. Heat the frying fat in a hot pan and fry the cordon bleu in it for 5-6 minutes over medium heat on both sides until golden.
In the meantime boil up the milk. Mash the potatoes with the fat. Add boiling milk and stir until smooth. Season with salt and nutmeg. Remove Cordon Bleu from the pan and degrease on kitchen paper.
Arrange cordon bleu with mashed potatoes and some beetroot salad on plates and garnish with parsley and lemon wedges. Serve the remaining beetroot salad separately.