In a large saucepan, melt the butter over moderately high heat. Add the onion and scallions and cook, stirring, until softened. Stir in the flour. Add the water, carrots, salt, pepper and thyme and bring to a boil. Cover, reduce the heat to moderate and cook until the carrots are just tender, about 8 minutes. Add the peas. Cover and simmer until the carrots and peas are tender, about 5 minutes longer. Serve hot.