How to Make It
Melt unsalted butter in a small saucepan over medium-high; add sweet onion; cook, stirring often, until tender and beginning to brown, about 5 minutes. Add habanero chile; cook, stirring constantly, 30 seconds. Remove from heat; transfer onion mixture to a blender.
Add blackberry preserves, fresh lime juice, and kosher salt; process until smooth, about 1 minute. Return onion mixture to saucepan; cook over medium, stirring often, until hot, about 2 minutes. Remove from heat; serve immediately. Or store in an airtight container in refrigerator up to 2 days. Reheat before serving.