Cook the Lentils: Rinse the lentils under cold water. In a medium pot, bring the lentils and water (or vegetable broth) to a boil. Reduce the heat to a simmer and cook for 20-25 minutes, or until the lentils are tender but still firm. Drain any excess liquid and set aside.
Roast the Vegetables: Preheat the oven to 400°F (200°C). Toss the carrots, sweet potato, parsnips, and red onion with olive oil, salt, and pepper. Spread the vegetables on a baking sheet and roast for 25-30 minutes, stirring halfway through, until golden-brown and tender.
Make the Garlic-Tahini Dressing: In a small bowl, whisk together the tahini, lemon juice, minced garlic, and water. Add more water as needed to reach a creamy, pourable consistency. Season with salt and pepper to taste.
Assemble the Salad: In a large bowl or on individual plates, layer the cooked lentils with the roasted vegetables. Drizzle the garlic-tahini dressing over the top and garnish with fresh parsley or cilantro.
Serve and Enjoy: Serve the salad warm, with extra dressing on the side if desired.