Preheat oven to 450 degrees. Stuff the cavity of the snapper with a sprig of rosemary and thyme. Place the fish on a cookie or sheet pan. In a large bowl mix the box of kosher salt and the citrus zests. Chop a sprig of the rosemary & thyme and mix it with the salt mixture and generously pepper. Add the water to the salt until you can form a semi-crumbly snowball, adding more salt and water if necessary. Pack this salt mixture around the fish (the crust shouldn't be more than a 1/2 an inch thick.) Bake in oven for 25 to 30 minutes. The shell should be solidified and have a faint golden color.;
Mix the 6 ingredients in a bowl and let macerate for 20 minutes;
Mix the citrus juices and mustard together and slowly emulsify olive oil into it, season.
Serve with fish