1. Lay out a sheet of baking parchment and place the cod (skin off) in the middle (one sheet per cod fillet) and season the cod with salt. Slice the lemon into thin slices and place on top. Add red onion (peeled and sliced) on top, along with the butter.
2. With a hand blender, blend the chives and olive oil together until vibrant green (you can otherwise mix them in a bowl with a spoon) and pour over the cod. Sprinkle the dill and a little extra salt and pepper on top, and tie the bag together using butchers string (or any ovenproof string) and place on a baking tray.
3. Bake in a preheated oven for about 15 minutes at 190 degrees. Remove from the oven and cut open the bag using a scissors (be careful of the steam).