Preheat oven to 220C/430F (200C/390F fan) degrees. Take a sharp knife and cut an 'X' into the side of each chestnut (this will stop them exploding in the oven!).
Put in a baking tray and roast for around 15-20 minutes, or until the shells have pulled back from the cuts you made and the inside has softened.
To make the crust
While the chestnuts are cooking, pop the breadcrumbs, peppercorns and mustard seeds into a food processor. Pulse for briefly until well combined but still coarse - don't worry the crust will break down further as we add more ingredients.
Add the parsley and pulse again.
Take the chestnuts out of the oven, wrap in clean kitchen towel and squeeze them to help break the shell. Leave to sit for 5 minutes before peeling to remove the shell and any pith.
Turn the oven down to 200C/390F (180C/350F fan) degrees.
Add the peeled chestnuts to the blender and pulse until the chestnuts are coarsely chopped.
Add 1 tbsp of olive oil and the juice of half a lemon and pulse through until you have a coarse paste. Set aside to prepare the salmon
To prepare the salmon
Before you begin cooking with your salmon, make sure that it is fully deboned. Run your finger along the edge of the salmon and use needle-nose pliers or strong tweezers to carefully pull each out at the same angle it is pointing.
Place the salmon skin side down on a well oiled and salted baking sheet (this will help to stop it from sticking). Oil and salt the flesh side of the salmon.
Crumble the crust mixture on top of salmon to cover evenly.
Arrange the lemon wedges around the tin.
Roast for around 20 minutes or until the salmon is cooked all the way through.
You can check if your salmon is done by poking a knife into it - the salmon should flake easily. If using a meat thermometer, you can also check that the thickest part of the fish reaches 50C/125F.