Heat butter in a medium pot over medium heat. Add scallions and salt and cook, stirring occasionally, until tender, 6 to 8 min. Add 2½ cups water and bring to a boil.
Reserve ½ cup of the peas and add the rest to the soup. Reduce heat to a simmer and cook 5 to 6 min.
Purée soup in batches until smooth; stir in lemon juice. Garnish with reserved peas.
Frozen peas are packed with sweet flavor and are flash-frozen when they are packaged, so they retain their goodness. Plus I typically have a bag of frozen peas in the freezer at all times for good measure! So this soup is easy to pull out of a hat...or freezer if you will!