Peel the potatoes and cook in plenty of salted water for about 20 minutes.
Melt the butter in a saucepan and sweat the flour in it, add the vegetable stock and the cream, bring to the boil and simmer for 10 minutes. Stir in the grainy mustard and bring to the boil again. Season the mustard sauce with salt, pepper and a pinch of sugar.
After 10 minutes, add the eggs to the potatoes and cook hard for another 10 minutes, quench, let cool down a little and peel. Drain the potatoes and let them evaporate a little.
For the spinach salad: put the rapeseed oil, balsamic vinegar and honey in a bowl and stir well, season with salt and pepper. Wash the spinach and drain well in a sieve. Mix the spinach with the dressing. Arrange the potatoes with mustard and spinach salad on plates. Serve sprinkled with parsley.