Melt the butter in a medium saucepan over medium-high heat. Add the onion and cook, stirring, until soft, about 3 minutes. Add the peas and water and bring to a boil. Reduce the heat to medium-low, cover the saucepan, and simmer until tender, about 4 minutes. Remove from the heat. Add the salt and mint and stir.
Using a slotted spoon, transfer the peas to serving plates. Serve hot.