Cook the lentils: Add rinsed lentils to a pot and cover with 2–3 inches of water.
Bring to a gentle boil, then lower to a simmer. Cook until tender but not mushy, about 18–25 minutes depending on the variety. Drain and spread on a tray to cool slightly.
Salt while warm: While the lentils are still warm, toss with a pinch of salt and a drizzle of olive oil.
This seasons them from the inside out.
Make the dressing: In a small bowl, whisk lemon juice, lemon zest, Dijon, garlic, honey, cumin (if using), 1/2 tsp salt, and several grinds of pepper. Whisk in olive oil until emulsified. Taste and adjust acidity or sweetness as needed.
Prep the vegetables and herbs: Finely chop red onion, cucumber, celery, tomatoes, parsley, mint, and dill.
Rinse capers briefly if very salty.
Combine: In a large bowl, add the lentils, vegetables, herbs, and capers. Pour over the dressing and toss gently until everything is coated.
Rest for flavor: Let the salad sit 10–15 minutes at room temperature so the lentils absorb the dressing. Taste and adjust salt, pepper, and lemon.
Finish and serve: Top with crumbled feta if you like.
Serve at room temperature with extra lemon wedges.