Place lentils and split-peas in a bowl and add cold water to cover. Set aside for 30 minutes. Drain.
Fill a large saucepan with water and bring to the boil. Add the lentils, split-peas, tumeric and chillies. Boil uncovered until the lentils and split-peas are tender (about 30 minutes). Drain, place in a bowl and mash coarsely. Set aside.
In a saucepan, heat oil over medium-low heat and add the mustard seeds. Cook until they crackle (about 30 seconds). Stir in the cumin seeds and cook until aromatic (about 30 seconds). Stir in garam masala and ground coriander.
Stir in the mashed lentils and peas, water, cream and tomato. Season with salt and bring to the boil over medium heat. Reduce heat to low and simmer, partially covered, stirring often for 4 minutes. Adjust the seasoning, stir in the fresh coriander leaves and serve hot.