Preheat the oven to 180C/350F and line a baking sheet with baking paper.
Season the inside of the trout with salt and pepper, and stuff it with a few sprigs of dill and parsley. Layer a few slices of lemon in the fish.
Drizzle the olive oil over the fish to coat it evenly. Season the outside well with salt and pepper.
Roast uncovered for 22 to 28 minutes, depending on the size of the fish; a 900gms fish takes about 22 minutes.
Once cooked, place the trout on a serving platter and carefully butterfly the fish, removing the central spine bone and any small bones.
Drizzle with more olive oil and pan juices, add the cooked lemon slices, and season with more salt and pepper. Scatter some dill leaves or parsley for garnish.
Boil the new potatoes until knife-tender, then drain and let them cool before slicing in half.
For the dressing, whisk together the mayonnaise, crème fraiche, caper brine, fresh lemon juice, and adjust the seasoning.
Toss the sliced potatoes with the fennel, radishes, spring onion, capers, herbs, and dressing before serving alongside the baked trout.
