1. Peel and chop the onions. Sauté them in hot butter until golden.
Peel the cucumbers, remove the seeds, cut them into 2cm pieces, and briefly sauté them with the onions.
2. Sprinkle with flour, let it thicken, then add vegetable broth. Add cream and mustard.
Mix well. Season with salt, pepper, and a touch of sugar.
3. Cut the fish fillets into strips. Season with salt, pepper, drizzle with lemon juice, and cook for 4 minutes with the cucumbers.
Rinse the dill, pat it dry, chop it, and sprinkle it over the dish.
Serve with mashed potatoes.