Place the split peas in cold water to soak while you start to prepare the rest of the curry. They don't need to soak, but I find it helps with minimising cooking time.
In a large heavy base saucepan, fry the onion, garlic, cumin seeds, fennel seeds, coriander seeds, chilli powder and turmeric in oil until the onion is translucent and the spices very fragrant.
Put the split peas into the saucepan. Add the pumpkin, diced tomatoes and 2 cups water. Cover and simmer until the split peas and pumpkin are cooked well, about 30 minutes. Add more water if required while simmering.