1.Boil, steam or microwave potato and peas, separately; drain. Rinse peas under cold water; drain.
2.Meanwhile, make dill dressing: combine olive oil, lemon juice, mustard, sugar and dill in screw-top jar; shake well.
3.Combine potato with ⅓ cup dressing in medium bowl.
4.Heat the water with lemon and peppercorns in medium frying pan; add fish. Simmer, covered, until fish is cooked as desired; drain. Cool slightly; flake into chunks.
5.Divide potato, peas, fish, spinach and mint among serving plates; drizzle with remaining dressing.