Wash and grate the lemon, cut in half and squeeze out the juice. Rinse the fillets, pat dry, season with salt, pepper and drizzle with lemon juice (about half the amount squeezed out). Finely chop the onion. Peel the cucumber, cut in half lengthways, scrape out the seeds and dice. Finely chop the dill.
Melt the butter in a pan, sauté the onion, add the cucumber and stew for 2 minutes. Deglaze with white wine. Stir in the cream cheese and season with dill, salt and pepper. Place the fish fillets on top and let steam with the lid closed for 5 minutes. Turn the fish fillets over and stew for another 5 minutes.