Preheat the oven to 180 degrees, convection 160 degrees, gas mark 3. Drizzle the fish fillets with lemon juice. Cut the vegetables into thin slices and bring to the boil once with the vegetable stock, olive oil, thyme, salt and pepper. Put the vegetables and the broth in a large baking dish.
Place the fish on the vegetables, season with salt and top with tomato slices, sage leaves and ham.
Pour in the fish marinade. Cook in the oven for about 20-25 minutes.