In a large saucepan, combine the lentils, water or broth, and bay leaf. Bring to a boil over medium high heat and then turn to low and cook for 15 to 20 minutes or until lentils are soft, but still slightly firm. Do not overcook or the lentils will be mushy.: You will notice a rising perfume from the pot as the lentils heat up, and the surface will move from still to a steady simmer with occasional small bubbles. Keep the lid slightly ajar if you see foam forming, and stir once or twice to ensure even cooking. The ideal texture is tender with a slight bite, almost al dente, so check a lentil at 15 minutes by tasting; it should yield but not collapse. Overcooking turns them into a puree, which loses the pleasant contrast with the raw vegetables, so drain promptly when they reach that tender but intact point. A common mistake is leaving them unattended on too high heat, which can cause them to split and become gluey.
While the lentils are cooking, make the dressing. In a small bowl or jar, whisk together the olive oil, lemon zest, lemon juice, Dijon mustard, honey or maple syrup, garlic, salt, and pepper. Set aside.: As you whisk, watch the dressing emulsify into a glossy mixture, the citrus scent bright and immediate. The mustard helps bind the oil and lemon into a cohesive emulsion, and the honey smooths the edges of the acid so the dressing feels rounded on the tongue. Smell the dressing to judge the balance; if the lemon dominates, add a touch more oil, if it feels flat, a pinch more salt or an extra drop of lemon will sharpen it. Let the dressing rest briefly to allow the garlic to soften and integrate. A common error is adding salt before tasting, which can lead to over salting once the dressing meets the lentils and vegetables.
When the lentils are done cooking, use a colander to drain the lentils. Discard the bay leaf. Rinse quickly with cold water. Transfer to a large bowl.: The rinse will stop carryover cooking and firm each lentil's surface, leaving a cooler, springier bite. When you drain, listen for the quick hiss of steam escaping, and see that the lentils shed excess starch, preventing them from clumping. Transferring them while still slightly warm helps the dressing soak in better, but ensure they are not hot enough to wilt the herbs or melt the feta cheese . One pitfall is draining too long and letting them sit dry, which makes them less receptive to the dressing.
Add the cucumber, red pepper, red onion, parsley, mint, and feta cheese, if using, to the bowl. Stir. Drizzle with dressing and toss until well combined. Serve immediately or let sit for 30 minutes so the flavors can meld.: As you add each item, note the burst of color and the fresh herb aroma that fills the bowl. Turning the salad gently keeps the feta cheese in small crumbles rather than mashed into a paste. Tossing by hand or with a large spoon ensures even distribution so each bite has a balance of textures. If the herbs clump, a few careful stirs will separate them; avoid vigorous mixing which can bruise delicate leaves. A common misstep is adding everything while the lentils are too hot, which will make the vegetables limp and mute the herbs' brightness.
Drizzle with dressing and toss until well combined. Serve immediately or let sit for 30 minutes so the flavors can meld: When the dressing hits the salad, you should see a light sheen on the lentils and vegetables, and the citrus aroma will lift. Toss gently to coat without crushing the vegetables or breaking the lentils. If you serve immediately, expect bright, crisp contrasts; if you rest it for 30 minutes, the flavors will integrate and the lentils take on more of the dressing's personality. Always taste before serving and adjust salt and pepper. A frequent oversight is over dressing; aim for a thin coating rather than pooling oil at the bottom of the bowl.