1 Scrub the potatoes thoroughly and put in a pot with just enough water to cover. Bring to a boil, add salt, cover and cook over medium heat until knife-tender, 20-25 minutes.
2 Drain the potatoes, rinse with cold water, peel and let cool.
3 Meanwhile, bring vegetable broth with vinegar, bay leaves, mustard seeds and peppercorns to a boil in another pot. Cover and cook over low heat for 10 minutes.
4 Rinse fish fillets and pat dry with paper towels. Cut into bite-sized pieces and place in the broth mixture. Cover and simmer over low heat for about 5 minutes.
5 Remove fish pieces from broth with a slotted spoon and place on a plate. Pour liquid through a fine sieve.
6 Cut cooled potatoes into slices. Heat the canola oil in a non-stick pan and cook over medium heat for about 20 minutes, turning occasionally.
7 Meanwhile, for the mustard sauce, peel the onion and chop finely. Melt the butter in a small pot and cook onion over low heat, covered, for about 5 minutes.
8 Sprinkle flour over onion and cook until golden. Add milk and strained cooking liquid gradually, while whisking. Bring to a boil, then simmer for about 5 minutes over low heat, whisking occasionally.
9 Stir both types of mustard into the sauce and bring to a boil. Stir in cream. Season sauce to taste with salt, pepper and sweetener and keep warm.
10 Rinse the parsley, shake dry, pluck off the leaves and finely chop.
11 Season potatoes with salt and pepper. Lightly salt the fish pieces, add to potatoes and gently mix. Heat until warmed through. Sprinkle with chopped parsley and serve with mustard sauce.