Heat 2 tablespoons of the oil in a flameproof casserole and gently fry the onion for 6–8 minutes until lightly browned. Add the garlic and herb and cook for about 2 minutes.
Drain the lentils and stir into the casserole with the tomatoes, sugar and stock. Bring to a simmer, then cover and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 10 minutes. Check over the fish for any stray bones and cut into 8 pieces. Season with salt and pepper.
Stir the parsley and anchovies into the casserole. Nestle the fish down into the lentils and drizzle the fish with the remaining oil. Re-cover and return to the oven for a further 25 minutes or until the fish is cooked through. Serve with spoonfuls of garlic mayonnaise.