Wash fish fillets and pat dry. Season both sides with salt and black pepper.
Sprinkle red onoin slices onto one side of the parchment paper to form a bed for the fillet. Place fillet on onions.
Top each fillet with equal amounts of tomatoes, olive and capers. Top each fillet with a lemon slice and sprigs of fresh or dried Thyme.
Seal parchment pouch as described.
Poach in oven for 15-18 minutes.
Transfer pouches to a plate and gentle pierce each with the tip of a small sharp knife. Use scissors to make a cut the length of the pouch, then accross at the center. Be careful and aware of the hot steam that will escape.
Serve pouch with steamed spinach or fingerling potatoes, or grilled fennel.