Heat the oil in a large skillet over medium heat. Add the cumin seeds and cook, stirring constantly, for 1 to 2 minutes, until the seeds start to pop. Add the onion and cook, stirring occasionally, for 5 to 7 minutes, until tender and translucent. Add the garlic and tomatoes and cook, stirring frequently, for 1 to 2 minutes, until the garlic is fragrant. Decrease the heat to low and stir in the lentils, ginger, cinnamon, paprika, harissa, and salt, then stir in the spinach. Cook, stirring frequently, until the spinach has wilted, adding water by the 1⁄4 cup (60 ml) if needed to prevent sticking. Stir in the pomegranate molasses. Taste and adjust seasonings if desired.