1. Wash and remove the stems from the sorrel, then pick the mint leaves.
2. Layer the hake fillets together, then tie them together to form a roast shape.
3. Melt the sorrel in a buttered casserole dish for 5 minutes.
4. Place the hake roast on the bed of sorrel and sprinkle with the washed and picked herbs. Drizzle with lemon juice and a splash of water, season with salt and pepper. Cook over very low heat, covered, for 20 to 25 minutes.
5. Check the doneness of the roast and return it to the casserole if necessary. Otherwise, remove it from the casserole, untie it, and remove the skin. Arrange the fillets on a warm serving dish.
6. Blend the cooking juices with the herbs. Pour into a saucepan, then thicken the sauce by incorporating the fresh cream. Heat gently for a few minutes until the sauce thickens, without bringing it to a boil.
7. Coat the hake fillets with the herb sauce and serve.
8. Serve with steamed potatoes or fresh pasta.