Prepare your fish: remove guts, scale it and trim tails and fins. Better yet, ask your fishmonger to do it for you. It's free!

Peel the onions and slice into half-rings. Place the onions into a large bowl and give them a few squeezes with your hands just hard enough to release some juice. Cut the lemon in half. Squeeze the juice from one half of the lemon and add to the onions. Add 1 teaspoon salt and 1 teaspoon pepper and mix well with your hand, gently squeezing the onions as you mix to release more juice.

Put a couple of pinches of salt and black pepper on each fish and rub all over, include the belly cavity.

Place the fish, belly up, into a container. Make sure the fish stays in that position otherwise the juices will run off. Stuff the onions and with the juice into the fish cavities (belly and gill area) just enough for the cavities to be able to close back up. Do not over-stuff. Cover and marinate in a refrigerator for 30 minutes. Set the remaining onions aside.

Preheat oven to 355F (180C).

On a large baking sheet lined with foil, make 4 equal beds of onions, each about the size of the fish. Spread fresh dill over the onion beds, then place rosemary sprigs on top. Place the fish on top of the onion and herb beds. Drizzle some olive oil on top of each fish.

Slice the remaining half-lemon and place the slices around the fish.

Cover with foil and bake for 35 minutes.

Remove the foil, switch the oven to broil and continue cooking for additional 3-5 minutes or until the fish acquires a nice brown color. You want to keep a close eye on the fish during broil as it can go from nice deep golden brown color to black char quickly.