Make sure there are no scales on the skin. Rinse well, the inside and outside.
Pat dry and place on a rimmed baking sheet.
Stuff the cavity of the trout with your choice of aromatics, if using.
Rub the skin of the fish all over with the olive oil (this will make it easier to remove the salt crust after roasting).
In a large bowl with a rubber spatula, mix the salt with the egg whites and water.
Spread enough of the salt mixture on the bottom of a large rimmed baking sheet to make a bed about 1/4 inch thick and roughly the same size as the fish.
Put the fish on top of the salt bed.
Using your hands, coat the fish with the remaining salt mixture to make a 1/4-inch-thick crust, molding it around the contours of the fish. (If the tail or head extends beyond the rim of the pan, it doesn't need to be covered with the salt mixture.).
Cook the trout 40 to 45 minutes.
Cook until the thermometer registers between 135°F and 140°F.Start checking at the lower end of the time range in the chart above. Let the fish rest in its crust for 5 to 10 minutes before serving.
To remove the salt crust, tap with a metal spoon to crack the crust and remove with spoon or fork. Use a pastry brush to remove any salt remaining.
Remove skin, separate top part of fish from bone to platter, remove bones and aromatics and remove lower part of fish from skin and put onto platter.