Add the sliced shallots, and crushed garlic and cook until soft but not brown. Stir in the diced tomatoes, cover with a lid and cook for 5 minutes until thickened.
For the duxelles, chop the mushrooms and shallot finely and cook without colour until tender and there is no liquid in the pan.
To prepare the garnish, cut the vegetables into thin matchsticks, cook in butter for 1 minute and reserve.
Cut two large sheets of parchment paper in half crosswise. Cut each half into a heart shape about 30 centimetres by 45 centimetreswide.
Place 2 tablespoons each of the mushroom duxelles and the tomato fondue next to each other in the centre of each parchment heart.
Place a fish fillet on top, skin side up. Place an equal portion of the garnish on top of each fillet. Top with a sprig of thyme and a drizzle of white wine.
Using a pastry brush, lightly coat the edges of the parchment hearts with beaten egg white. Fold the heart shape in half to cover the fish and press the edges together to seal. Using a pastry brush, lightly coat the sealed edges with egg white.
Make a series of short folds along the edge to reinforce.
Place the packets on a baking tray in the preheated oven at 220oC .
Bake for about eight minutes or until the packets puff up.
Serve immediately, cutting open the packet at the table so the delicious aroma is released.