To make the dal: combine split peas, water and turmeric in a large saucepan. Bring to a simmer. Partially cover, reduce heat to medium-low and cook until split peas are tender, 40 to 45 minutes.
Meanwhile, heat oil in a large non-stick frying pan over medium heat. Add cumin seeds and cook, stirring, until fragrant, 10 to 20 seconds. Add onion and cook, stirring often, until softened, 2 to 3 minutes. Add ginger, garlic, fenugreek, if using, and chilli. Cook, stirring, until fragrant, 20 to 30 seconds. Add tomatoes and cook until most of the liquid has evaporated, 5 to 10 minutes.
When split peas are tender, stir them and any remaining cooking liquid into tomato mixture, along with the spinach. Cook for 2 to 3 minutes to blend the flavours.
To make the raita: mix the yoghurt, ground cumin and mint in a small bowl. Serve the dal with raita.
Leftover dal will keep, covered, in the refrigerator for up to 2 days. Reheat on the stovetop or in the microwave, adding a little additional water, if necessary.