In large heavy saucepan, heat oil over medium heat. Add cumin and mustard seeds and sauté until they begin to pop, about 1 minute. Immediately add onion, ginger, turmeric, and ground cumin. Add a splash of water and sauté for about 3 minutes, or until onion is soft but not browned. Stir in diced tomatoes and their juices, 4 cups stock, lentils, and cinnamon stick and bring to a gentle boil. Cover, reduce heat, and simmer gently for 20 minutes. Stir occasionally, adding a little more stock for a thinner consistency. Stir in coconut cream for added richness if you wish. Cover and continue simmering for a few more minutes, or until piping hot. Remove cinnamon stick and stir in lime juice and maple syrup. Add salt and pepper to taste, if desired.