Melt butter in a medium saucepan over medium-high heat. Add onion and cook, stirring, until soft, about 3 minutes. Add peas and 1/2 cup water and bring to a boil. Reduce the heat to medium-low, cover saucepan, and simmer until tender, about 4 minutes.

Remove from heat. Add salt and stir. Using a slotted spoon, transfer peas to serving plates. Serve hot.