In a large pot, combine water,lemon, onion and pepper, and boil for two minutes. Bring to a simmer and add salmon. Cook for about seven minutes or until cooked through. Remove salmon from pot, place in a bowl and let cool.
Using a fork, flake salmon into small pieces and add red onion, dill, lemon juice, yogurt and horseradish, and fold together. Season to taste.
Cover four pieces of bread with a layer of cucumber slices. Divide salmon mixture among them. Top each with more cucumber slices and a slice of bread.
Per serving: 368 calories, 29 g protein, 16 g fat (4 g saturated fat), 26 g carbohydrates, 4 g fibre, 56 mg cholesterol, 307 mg sodium