For marinated kipflers, cover potatoes with salted cold water in a large saucepan, bring to the boil and cook until tender (10-15 minutes). Meanwhile, blend remaining ingredients in a bowl with a hand-held blender. Drain potatoes, peel and, while still hot, add to marinade, stir to coat and marinate (2-3 hours).
Melt butter in a small frying pan over medium-high heat, add sourdough cubes and toss occasionally until golden and crisp (3-5 minutes), then drain on paper towels.
Grill calamari over high heat without turning until coloured on one side (2-3 minutes). Set aside.
Remove potatoes from marinade and thinly slice, and place in a bowl with radicchio, parsley, shallots, capers and cooked calamari. Drizzle with potato marinade and toss to combine. Season to taste, add croûtons, toss again and serve.