Adjust a rack in the center of your oven. Heat the oven to 400 degrees F.
Pat the fish. With a sharp knife, make two slits on both sides of the fish. Rub the fish with kosher salt and black pepper on both sides, inserting the seasoning through the slits and in the fish cavity.
Stuff the fish cavity with the sliced onion and lemon.
Roast in the center rack of your heated oven for 5 minutes on one side, then turn over and cook for another 5 to 7 minutes or until the fish is cooked and flakes easily. Turn the broiler on and place the fish about 6 inches from the heat source for 3 to 4 minutes or until the skin chars.
While the fish is cooking, prepare the Greek ladolemono sauce.
As soon as the fish is finished, remove it from the oven. Transfer the fish to a serving platter and drizzle immediately with as much of the ladolemono sauce as you like, making sure to add some of the sauce all over the cavity part as well. Toss the cherry tomatoes with a little salt and spoon them over the fish (you can drizzle a bit of the sauce again all the tomatoes). Finish with fresh dill. Serve immediately.