Add the rinsed red lentils, onion, carrots, garlic, ginger, diced tomatoes, cumin, turmeric, garam masala, coriander, optional cayenne, salt, and oil to a 4- to 6-quart slow cooker.

Pour in the boiling water or vegetable broth and stir well, making sure the lentils are fully submerged and spread evenly across the bottom.

Cover and cook on HIGH for 35 minutes, keeping the lid closed so the slow cooker holds heat.

Open the lid, stir the dal, then fold in the baby spinach. Cover again and cook on HIGH for 5 more minutes, until the spinach wilts and the lentils are soft but still lightly textured.

Stir in the lemon juice. If the dal is thicker than you like, add a splash of hot water; if it is thinner, let it sit uncovered for 3 to 5 minutes. Taste and add more salt if needed.

Serve warm over cooked rice or with naan, topped with cilantro if using.