Step 1Rinse the fish thoroughly under running water; pat dry with paper towels. Generously sprinkle with salt. Place the sliced lemons in the cavity and layer the herbs and fennel fronds on top of the fish.
Step 1Heat the oven to 450 degrees. Place a large roasting pan inside to heat for 5 minutes.
Step 2Remove the pan and brush the oil on the bottom, then add the potatoes, fennel, garlic, onions, anchovies, rosemary and thyme. Stir to coat. Roast for 15 minutes. Push the vegetables to one side and place the fish in the pan and scatter the butter around the vegetables and fish. Roast for 10 minutes. Carefully turn the fish over with a spatula. Continue roasting until the fish tests done, about 15 minutes; to check, insert the tip of a knife near the backbone of the fish at the thickest point. The fish should be opaque and pull away easily from the bone. Add the tomatoes the last 5 minutes of cooking.
Step 3Remove the fish and place on a warm serving platter. Rub the fish with the cut head of garlic carefully to avoid breaking the skin (this imparts a nice garlic flavor). Squeeze the half lemon over the fish, and sprinkle with the Sherry vinegar and fleur de sel to taste. Place the vegetables from the pan around the fish and sprinkle with the parsley.