Place the salmon or trout fillet on a chopping board. Crumble the feta cheese and spread evenly over the salmon fillet. Squeeze the frozen spinach to remove as much moisture as possible then sprinkle over the feta cheese.
Place three sheets of puff pastry on top of each other and roll out to form a large rectangle. Place the salmon fillet in the centre so that you can fold the puff pastry around it. There should be enough puff pastry to cover the entire salmon fillet. Remove any excess pastry before sealing. Use water as “glue” to seal where the puff pastry overlaps.
Make four holes evenly spaced over the top of the puff pastry.
Place the salmon in the puff pastry on the bottom shelf of the oven and bake for 20-25 minutes until the pastry is golden on top.
Make the Sandefjord butter by adding the cream to a pan and boiling until it leaves a residue on a spoon. Melt in the butter with grated lime zest, salt and pepper. Add the mint leaves and rings of green chilli, and serve.
Serve the salmon en croûte with the Sandefjord butter.