Heat the oil over medium heat in a dutch oven or large saucepan. Add the mustard and cumin seeds and cook for 1 minute.
Add the ginger, hot pepper, garlic, coriander, turmeric, pepper and cinnamon, and cook for 1 minute more.
Add the onions and carrots and cook for 3-4 minutes, stirring constantly.
Add the lentils, rhubarb and 6 cups of water. Bring to a boil, reduce heat and simmer until lentils and vegetables are tender, about 20-25 minutes. Add salt to taste halfway through cooking time. If the dahl gets too thick, add a little extra water.
Serve with brown rice, a dollop of yogurt, or some sauteed greens.
Serves 6.