While potatoes are cooking, melt butter in large frying pan over low heat.
Add scallions and garlic. Cook, stirring, until fragrant.
Increase heat to medium-high.
Add half the greens and saute, stirring frequently, until greens begin to wilt, about 2 minutes.
Stir in remaining greens and repeat.
Add broth, salt, and pepper.
Stir in lemon zest and juice.
Simmer, stirring occasionally, until greens soften, about 6 minutes.