Place all of the mixed lentils ingredients into the slow cooker and stir to combine. Place the lid on the slow cooker and cook for 6-8 hours on low. If the consistency is too thick, add additional water. Meanwhile, make the curry sauce. In a large saucepan, heat the oil over medium-high heat. Add the onions and sauté until tender-crisp, about 5-6 minutes. Stir in tomatoes, ginger and garlic paste and continue to cook until tomatoes are tender, about 7-8 minutes. Add the garam masala and chili powder, if desired, and cook for an additional 1-2 minutes. Add the curry sauce to the slow cooker and cook an additional hour on low. Serve with Nan or rice.