Put the lentils, garlic, sage, and olive oil in a small pot, along with 2 cups cold water, and set it over medium heat. Let the water come to a simmer, then turn the heat to low and cook the lentils in a gentle simmer until tender, about 25-35 minutes. Drain and discard the sage and garlic.
Mix together the ground coriander and cumin in a small bowl. Mash the garlic clove to a paste with 1 teaspoon salt. In a small bowl for your dressing, combine the mashed garlic, tahini, 3 tablespoons of the lemon juice, 2 tablespoons of the olive oil, 1 teaspoon of the ground coriander-and-cumin mixture, and 2 tablespoons water. Stir well and taste, adding more lemon if desired.
Place onion slices in a medium bowl, sprinkle with salt, lemon juice, olive oil, and cilantro or parsley. Toss well, then crumble in the cheese and gently toss again.
In a large mixing bowl, toss the lentils with the drained chickpeas, preserved lemon rind (if using), and 1 teaspoon flaky sea salt. Pour in the tahini dressing and toss well, then stir in the onion-feta mixture.
Arrange the salad on a bowl or platter. Sprinkle with sesame seeds and some of the remaining spices. Serve.
