Pre-heat a gas grill or prepare a fire in a grill.
In a bowl whisk together the mustard, garlic, balsamic vinegar, salt, seasoning mix, and black pepper until blended. Slowly whisk in the olive oil until it is incorporated.
Pour the mixture over the fish to completely coat with the marinade. Let stand at room temperature for 30 minutes.
Place the fish, on a well-oiled grill rack. Arrange the lemon slices over the top of the fish, then cover with the sprigs of fresh dill.
Place the fish on the grill, cover and cook for 5 minutes. Baste with any remaining marinade. Cover and cook for an additional 30 minutes until skin is crispy and charred.
Carefully remove the fish from grill using two wide spatulas. Remove the lemons and the dill. To serve, gently scoop the flesh of the fish out of the skin. Garnish with fresh dill and lemon if you wish.