Heat the oven to 200°C/180°C. Arrange the fennel slices in a shallow roasting tin or dish big enough to hold the fish. Drizzle with 2 tbsp of the oil, season with salt and pepper, then roast for 15 minutes until starting to soften.
Slash one side of each fish 3 times with a sharp knife, then tuck a bay leaf into each slash. Remove the fennel from the oven and scatter the sourdough over it. Sit the fish on top, slashed-side up, then sprinkle with the sliced preserved lemons. Drizzle the fish with the brine, 2 tbsp of the olive oil and the vermouth, then season with black pepper only. Return the dish/tin to the oven and roast for 20 minutes or until the flesh is opaque and comes away easily from the bones when you test the fish with the point of a knife.
Meanwhile, mix the reserved fennel fronds, parsley and dill with the sliced olives, remaining olive oil and the lemon juice. Spoon the herb/olive mix over the fish and serve with buttered potatoes or more sourdough.