Cook lentils in a saucepan of unsalted boiling water until just tender (10-12 minutes), drain, refresh, drain well and place in a large bowl. Combine onion, garlic and lemon juice in a bowl and stand until onion softens (5 minutes). Stir in oil, vinegar and sumac, add to lentils with herbs, and toss to combine.
Blanch asparagus until just tender (1-2 minutes), refresh, drain and set aside. Cook eggs in a saucepan of boiling water until soft-boiled (7 minutes), cool under cold running water, then peel and halve. Top lentil salad with asparagus and eggs, season to taste and serve scattered with almonds, feta and sumac.
Note: Persian red lentils are available from select delicatessens; if they're unavailable, substitute small green lentils.