Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Wash the asparagus and peel the lower third and cut off the woody ends. Cut the asparagus into pieces of about 5 cm and cook in little boiling salted water for about 8 minutes.
Wash the watercress, dab dry and scrape the leaves off the stalks. Wash the fish, dab dry and cut into bite-sized pieces. Mix lemon juice, mustard, turmeric and some salt.
Add oil. Drain the potatoes, rinse with cold water and remove the skin. Cut the potatoes into cubes. Heat the fat in a frying pan and fry the catfish for about 3 minutes while turning. Season with salt and pepper and take out.
Deglaze the roast with the mustard sauce, add the potatoes and drained asparagus and toss briefly. Fold in fish and 3/4 of the watercress, season with salt and pepper and arrange on a plate.
Serve garnished with remaining watercress and lemon.