Stir the saffron into the wine and set aside. Heat 2 tablespoons olive oil in a medium saucepan over medium heat. Add the fennel and onion and cook, stirring, until slightly softened, about 5 minutes. Add the garlic and cook, stirring, 1 minute. Add the saffron-wine mixture and olives and simmer until the liquid is reduced to about a tablespoon, about 3 minutes. Add the vegetable stock and cook until slightly reduced, about 2 minutes. Stir in the tomatoes. Season with salt and pepper; stir in the parsley.
Meanwhile preheat the broiler. Pat the fish dry with paper towels. Rub the skin with 3 tablespoons olive oil and season with salt and pepper. Heat 3 tablespoons olive oil in a large ovenproof skillet over high heat. Add the fish skin-side up and cook until golden on the bottom, 1 to 2 minutes. Transfer the skillet to the broiler. Broil until the skin is slightly charred, 6 to 8 minutes.
Divide the vegetables and liquid among 4 dishes. Top with the fish skin-side-up. Garnish each dish with fennel fronds and finish with gray salt.
Marc cooks the snapper on his wood-fired grill, then finishes it in the wood-burning oven. To get a similar effect at home, start the fish in a skillet, then broil.