Preheat the oven to 400°. In a large bowl, stir the egg whites with the 7 cups of kosher salt, the fennel and 3/4 cup of water until the mixture resembles moist sand. Spread a scant 1/2-inch-thick layer of the salt in an oval baking dish large enough to hold both fish. Pat the salt into a neat oval and place the fish belly to belly, on top. Pack the remaining salt on top of and around the fish. Poke a hole into the crust at the thickest part of the fish, behind the head. Bake for 25 to 30 minutes, until an instant-read thermometer inserted in the hole registers 135°. Transfer the dish to a warm place and let the fish rest for 10 minutes.