To prepare lentils: Combine water, lentils, garlic powder, 1/2 teaspoon coriander, cumin, allspice, 1/4 teaspoon salt and sumac (if using) in a medium pot. Bring to a boil. Reduce heat to maintain a simmer, cover and cook until tender, 25 to 30 minutes.
Uncover and continue simmering until the liquid reduces slightly, about 5 minutes more. Drain. Stir in lemon juice and 1 teaspoon oil.
Meanwhile, to prepare vegetables: Heat oil in a large skillet over medium heat. Add garlic and cook until fragrant, 1 to 2 minutes. Add roasted root vegetables and cook, stirring often, until heated through, 2 to 4 minutes. Stir in kale (or beet greens) and cook until just wilted, 2 to 3 minutes. Stir in coriander, pepper and salt.
Serve the vegetables over the lentils, topped with tahini (or yogurt). Garnish with parsley, if desired.