Fish en papillotte (fish in a parcel) is a brilliant way to cook fish to perfection. In this recipe, I'm also cooking some potatoes and onion and some asparagus.
Preheat the oven to 210℃ (410℉). Peel the onion and potato, and slice them as thinly as you can (you can use a mandoline for this if you have one). Trim the ends off the asparagus to make them the same length as your fish fillets.
Cut two sheets of greaseproof paper. They should be about three times as wide as the fish, and three times as long. As you build the first package, remember to not use more than half of the potato, onion and asparagus. Now you need to build a stack of ingredients, the same size as the fish. Place a layer of onion centred across the length of the paper but with the top edge aligned to the vertical middle, so that when you fold the parcel up you'll have extra paper to fold under the parcel. Place a layer of potatoes on top of the onions. Season with salt and pepper, and place some small blobs of butter on it. Repeat the layers twice more. Place a single layer of asparagus, add seasoning and butter, and finally, the fish goes on top with more seasoning, butter, and the fennel seeds.
To wrap the parcels, fold the bottom edge up, and the fold in the sides. Make a sharp crease on the sides, and fold the top flap down and under the parcel. You can tie with string if you like, but I generally don't bother.
Place the parcels on a baking sheet in the oven, and bake for 20-25 minutes.
Place the parcels on plates for your guests to unwrap at the table. It's worth doing this just to hear the oohs and aahs as the aroma is unleashed. Serve with a lemon wedge and some parsley sauce.