Pre-heat the oven to 400°F Fry the finely chopped onion in butter in a pan until soft and golden (but not burnt).
Add the bacon and tarragon and cook for another two minutes; then stir in the lemon juice.
Place a large piece of buttered kitchen foil in an ovenproof dish, place the salmon fillets on this and cover with the onion and bacon mixture. Fold over the foil and seal to make a parcel. Bake for 15-20 minutes.
Serve with a teaspoonful of heavy cream or creme fraiche and fresh vegetables.