in large saucepan, heat oil over medium-low heat and saute the onion until it turns dark brown, about 8 minute add coriander, garlic, ginger, turmeric, cayenne, and cumin and stir for 2 minute add tomato and stir over low heat until it disintegrates.
add peas and mix well. pour in the water, add salt and sugar, and bring to a boil. turn heat down to low, cover, and simmer until peas are cooked through, about 10 minute (20 minutes if using dried peas). stir in coconut milk and simmer uncovered for another 8-10 min., again allowing the flavors to come together.
add cilantro and lemon juice, simmer for 1 minute more, and remove from heat. serve hot.