Rinse the lentils well, then add them to a pot with the vegetable broth and water. Bring to a boil over high heat, then reduce the heat to medium low and simmer for 20 to 30 minutes, stirring occasionally, until the lentils are tender. Drain and set aside to cool.
Warm the oil in a small pot over medium heat until it begins to shimmer. In the meantime add the garlic, cumin, coriander, fennel, and chili flakes to a heat-safe bowl or dish. Pour the hot oil over the garlic and spices; they should immediately start to sizzle. Swirl the mixture gently while it cooks; once the sizzling stops stir in the maple syrup, lemon juice, and salt. Set aside.
Add the cooked lentils and beets to a large bowl and pour the dressing on top of them. Gently mix together, until everything is evenly coated in dressing. Add the arugula, parsley, onion, and mint to the bowl and gently fold into the lentil mixture. Sprinkle the hazelnuts and feta on top.
Serve immediately, or chill the salad in the fridge for 15 to 20 minutes if desired. Store any leftovers in an airtight container in the refrigerator for up to 5 days.
