Heat oven to 400 F (200 C).
In large saucepan, place potatoes and cover with water. Bring to a boil and heat until potatoes are just slightly tender. Drain and, when cool enough to handle, slice potatoes in half.
Toss potatoes with 2 tsp (10 mL) oil and 1/4 tsp (1 mL) salt. On rimmed baking sheet, spread potatoes out, and roast in preheated oven until tender and crispy, stirring once, about 25 minutes.
Toss radicchio with 1 tsp (5 mL) oil and 1/4 tsp (1 mL) salt. Place radicchio on top of potatoes and heat until leaves have wilted and are browned in a few places, about 4 minutes.
On separate greased or parchment paper-lined baking sheet, place trout and bake for 12 minutes, or until just cooked through in the centre. Let rest for 5 minutes. Then peel off skin and open up trout. Gently remove bones in one piece. Then gently break apart flesh into large chunks.
For dressing, in bowl, whisk together kefir, cider vinegar, turmeric, garlic, honey, salt, black pepper, and cayenne. Slowly drizzle in olive oil, whisking constantly.
To serve, divide potatoes, radicchio, trout, peppers, and parsley among serving plates. Spoon on kefir dressing and sprinkle walnuts and capers overtop.