Combine the apple butter, vinegar, chopped onion, honey, and gochugaru in a low, wide pan.

Set the heat to medium and bring to a simmer. Cook, stirring regularly, until onions are soft.

Taste and add salt and pepper to taste.

Scrape sauce into a deep measuring cup or bowl and puree it with an immersion blender until smooth.

Funnel into a jar and refrigerate.