Step 1

Heat a large cast-iron skillet over high until smoking hot. Add scallions and cook, turning occasionally, until evenly blackened, 8–10 minutes. Transfer to a cutting board and let cool; coarsely chop.

Step 2

Mix scallions, lime zest, and lime juice into butter in a medium bowl until evenly blended; season with salt and pepper.

Step 3

Do Ahead: Butter can be made 2 weeks ahead. Cover and chill.