Peel and slicethe potatoes to ¼ inch (8mm) thick, and place on a large baking sheet.
Drizzle with 4 Tbsp of salmoriglio sauce and toss until well coated. Season with salt and pepper. Spread the slices evenly over the whole surface of the pan to insure the potatoes turn deeply golden.(Optional: scatter garlic cloves among the potatoes). Bake for 20 minutes.
Meanwhile prepare the fish:
Rinse the fish and pat completely dry.
Place the fish on a cutting board. Depending on size, make 3 to 4 diagonal incisions into the thickest part of the flesh of each side with a sharp knife, cutting through to the bone to help it cook evenly throughout.
Season with salt and pepper, inside and out, and drizzle a teaspoon of salmoriglio sauce on each side of the fish, into the slashes, and inside the cavity. Stuff the belly with a handful of rosemary, bay leaves and a few slices of lemon.
20 minutes into cooking the potatoes:
Scrape the potatoes off the bottom of the baking pan with a spatula, toss, lightly sprinkle with more salt, and make sure the potatoes are evenly distributed on the surface of the baking pan.
Place the fish on top of the potatoes, and cook for about 20 to 30 more minutes, or until a thermometer reads 145°F (62°C) in the thickest part of the fish; or if the flesh comes off the middle bone easily with a fork. For great texture, do not overcook.
To serve, drizzle with more salmoriglio sauce, andgarnish with some fresh herbs and a lemon wedge. Serve more salmoriglio at the table.
What Size Fish to Buy?
For a single portion, around 14 ounces (almost a pound, 400g) of whole fish is considered a good size. This yields about 50% flesh and 50% head, guts and bones.
For two people to share, I would recommend a whole fish between 27-35 ounces (1¾-2¼lb, 800g-1kg) in total weight.
A single large fish or multiple smaller fish? Get whatever looks best. If you happen to come across a great specimen, say 3½-4 pounds (1.5-1.8kg), it would make a great presentation for four.
Keep in mind that smaller fish will need less time in the oven, while larger fish will need to cook longer.
Instructions for the Fishmonger - The fish must be scaled, gutted, cleaned and pat dried before roasting. I highly recommend to ask the fishmonger do this, as it's very messy to do at home — especially the scaling, as the scales tend to fly everywhere, literally! So, if you want to do yourself a favor, ask the fishmonger to do it for you — it's part of their service.
Other Fish You Can Use with This Recipe - red snapper, sea bream (orata), salmon (a small one), tilapia, and trout are a few good options.