For the croutons, preheat the oven to 300°F (180°C). In an ovenproof dish, toss the diced bread with oil.
Bake in the oven 8 to 10 minutes, or until almost crispy. Cool on paper towel. Sprinkle with salt.
For the potato salad, boil the potatoes until tender. Meanwhile, make the dressing by mixing mayonnaise and sour cream, then mixing in garlic, shallots, lemon zest and chives.
Add lemon juice and season with salt and pepper. Strain the potatoes and set aside for a minute to steam dry and cool slightly – but not entirely.
Cut potatoes in two or four – they are difficult to work with when hot but uneven pices is fine. Fold warm potatoes into the dressing. Let sit for 10 to 20 minutes to sop up the dressing before serving.
For the blueberry vinegar, combine pepper, sugar, white wine vinegar and blueberries – squeezing the berries slightly to give off colour – and stir.
To serve, arrange fish and potato salad neatly on a plate, garnish with a handful of croutons, and sprinkle with blueberry vinegar.