Preheat oven to 50°C. Bring fish stock with oil and thyme to a boil. Add trout fillets, reduce heat and let it sit 5 minutes. Remove fillets and keep warm in oven. Boil down fish stock by half, pour in cream, boil down some minutes and mix well with hand-held blender.
Polenta: Bring to beef stock a boil, grated rind of lemon and butter. Stir in polenta and simmer about 40 minutes, stirring regularly. Add cream and simmer for a further 5 minutes. Season to taste with salt and pepper.

Tip: Cook polenta the day before and fill into an oiled baking dish. Before serving, cut polenta into slices and fry them with butter both sides.