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.
YIELD: About 3 cups, serving 4 to 6