Warm the oil in a large Dutch oven or other heavy pot set over medium heat. Add the onion, cumin, coriander, and turmeric, and cook, stirring, until the onion begins to soften, about 5 minutes. Add the tomatoes, greens, a large pinch of salt, and water. Stir everything well to combine, cover, and simmer, until the greens are very tender and soft, about 15 minutes. Turn off the heat, stir in the lemon juice, season the greens to taste with salt, and serve immediately. Leftovers can be stored in an airtight container in the refrigerator for up to a few days and rewarmed in a heavy pot set over low heat (stir while you heat).