Add the onion, garlic, jalapeño and scallion whites to the pot and cook over moderate heat until softened but not browned, 7 to 8 minutes. Whisk in the flour, cumin and coriander and cook for 1 minute, whisking constantly. Whisk in the fish stock and bring to a boil. Reduce the heat to moderately low and simmer the soup for 15 minutes, stirring occasionally. Add the peas, scallion greens and the remaining 1/2 cup of cilantro and bring the soup to a simmer over moderate heat.