Cook the potatoes in boiling salted water for about 10 mins, until tender. Drain really well, then return to the pan and steam dry for a few mins. Remove from the heat, add the horseradish, then mash together.
Gently stir in the flaked trout, lemon zest and half the juice with some seasoning. When the mixture is cool enough to handle, make 8 patty-shaped cakes, dust each one lightly with flour, then chill for 5 mins to set.
To make the mushy peas, put the peas in a pan with the stock and bring to the boil, reduce the heat and cook for 3 mins until tender. Transfer to a food processor, or use a stick blender, and whizz for 30 secs to a crush. Tip them back into the pan, add a squeeze of lemon juice and some seasoning, and keep warm.
Heat the oil in a frying pan until hot, cook the fish cakes for 3-4 mins on each side until golden and crisp – you may have to do this in batches. Serve with mushy peas and some extra lemon wedges on the side.