Peel, wash and dice the potatoes. Cook in boiling salted water for about 12 minutes. Hard boil eggs for about 10 minutes. For the sauce, peel and finely chop the onions. Steam them in hot fat until golden brown.
Sweat flour in it. Add cream and milk while stirring constantly, bring to the boil. Season to taste with salt, pepper and mustard. Wash parsley, chop very finely and stir into the sauce. Quench the eggs cold, peel them and halve them lengthwise.
Arrange eggs, potatoes and sauce on plates. Wash the sprouts and place them on the plates. Serve garnished with parsley.