Toss beets with olive oil, paprika, coriander and salt. Spread out in a single layer on a large baking sheet and bake for 25-30 minutes, until tender. Let cool.
Heat olive oil in a medium saucepan over medium high heat. Add garlic and lentils and cook for 1 minute. Stir in broth and salt and cook until lentils are tender, about 20-25 minutes, stirring occasionally. Remove from heat and let cool slightly. Stir in vinegar.
In a large bowl, toss together cooked beets, lentils, onion, walnuts, cucumber, feta and greens. Mix well, salt and pepper to taste.