Rinse the lentils in a fine mesh strainer under cold water to remove dust and dirt. Check for discolored or broken lentils and discard them. Line a large baking sheet with parchment paper and coat with non-stick cooking spray.
Transfer the rinsed, drained lentils to a medium pot and fill with approximately 3 cups of water (enough to cover them by about 1 inch). Bring to a boil, then immediately reduce the heat to low. Cover and simmer until the lentils are tender (not mushy), about 15-25 minutes. Check for doneness occasionally.
Drain in a fine-mesh strainer, then spread them over the prepared baking sheet to cool.
As the lentils cook, whisk the olive oil, lemon juice, Dijon mustard, garlic, honey, salt, and cumin in a medium bowl—season with additional salt, lemon juice, and mustard to taste.
Add the cucumber, tomatoes, onion, and parsley to a large mixing bowl. Drizzle with about one-third of the dressing and gently toss to coat. Add the cooked and cooled lentils to the bowl and drizzle with the remaining dressing. Toss until just combined with the vegetables.
Serve immediately or let it sit for 30 minutes to let the flavors meld. Before serving, sprinkle with crumbled feta or toss it directly in the salad. Serve with fresh lemon wedges, if desired.