Rinse the lentils in a sieve and cook in boiling water for about 25 minutes until al dente. Drain, drain and let cool.
Pluck the salmon into pieces, core and dice the pepper. Halve the cucumber, remove the seeds and dice the pulp as well. Finely chop the peeled onion, rinse the dill, shake dry and pluck the fine leaves from the stem.
For the dressing, rub the zest of the lemon and squeeze out the juice. Mix two tablespoons of lemon juice with the zest, mustard, salt, pepper and oil.
Mix the lentils and vegetables with the dressing, then carefully fold in the salmon and dill, season to taste.