In a baking tin large enough to fit the whole fillet, lay down sheets of aluminium foil with enough overlap to wrap the fillet up like a parcel. Add the onions in a layer and then place the salmon fillet (if you use the skin, place the skin side down), on top of the onions. Rub the fillet with salt and pepper. Lay the orange slices and star anise evenly spaced over the top of the fillet, and place 2 lemongrass which have already been cracked into each side.