Preheat oven to Gas 4/180°C/Fan 160°C.
In a medium pan, sauté the onions and garlic in the oil for 3 to 4 minutes until they begin to soften. Mix with the potato and fennel slices, and the herbs. Season with salt and pepper.
Divide the vegetables between four small or one large ovenproof dish. Bake for 15 minutes, turning once.
Stir in the lemon juice and zest. Place one fillet over the vegetables in each baking dish. Top with lemon slices. Cover with foil and continue baking for 10 minutes, or until the fish is just cooked through.
Serve with mixed vegetables.
The lemon and oil make a dressing.