Heat the oil in a large pot over medium heat. Add the sliced mushrooms and onion, and cook, stirring often, for 5 minutes until the mushrooms and onions have softened.
Add the ginger, garlic, curry powder, turmeric, cumin, and paprika, and cook for 1 minute more, until fragrant. Add the lentils and water and cook for 10 - 15 minutes until the lentils are tender.
In a measuring glass or small bowl, whisk together the coconut milk and cornstarch. Stir into the pot and cook for a few minutes more until it thickens slightly.
Turn off the stove and stir in the spinach, add a squeeze of lemon and add salt and pepper to taste. Serve hot with rice or quinoa, or a side of naan.
Notes
For oil-free: simply omit the oil and do a water or broth sauté.