Preheat grill on high. Place bread on a baking tray. Cook under the grill, turning occasionally, for 5 mins or until crisp.
Meanwhile, place the sugar snap peas in a large heatproof bowl. Cover with boiling water. Set aside for 2 mins or until tender. Refresh under cold water. Drain well.
Combine the bread, sugar snap peas, apple, watercress, rocket, zucchini, fennel and onion in a large bowl. Combine the buttermilk, mustard and vinegar in a screw-top jar and shake to combine. Pour half the dressing over salad and toss to combine.
Heat a large non-stick frying pan over high heat. Drizzle fish with oil. Cook one-third of the fish for 4 mins each side or until brown and cooked through. Transfer to a plate and cover with foil to keep warm. Repeat, in 2 more batches, with the remaining fish.
Divide the salad and fish among serving plates. Drizzle the salad with remaining dressing. Sprinkle with walnuts. Season with pepper to serve.