Butter (or spray with non-stick spray) one side of a large sheet of aluminum foil (double the size needed to hold the salmon fillets in one layer); place in a baking dish (butter side up) large enough to hold the salmon (again, in one layer).
Fry the finely chopped onion in butter in a frypan until soft and golden but not brown.
Add the bacon and tarragon and cook for another two minutes.
Stir in the lemon juice and remove from heat.
Place the salmon fillets on the foil in the baking dish.
Cover each fillet with some of the onion/bacon mixture and drizzle any pan juices over as well.
Fold the foil over the fish and seal to make a parcel.
Bake for 15-20 minutes, or until fish flakes easily with a fork.
May be served with a dollops of thick cream or creme fraiche (but I personally don't think it needs it!).