Meanwhile, in a large saucepan, heat the canola oil. Add the onion and cook over low heat, stirring, until softened, 6 minutes. Add 6 cups of water and bring to a boil. Simmer over moderately low heat until the water is reduced to 4 cups, 20 minutes. Add the peas and simmer for 4 minutes. Add the spinach and cook until wilted, about 1 minute.