Combine all ingredients in a multicooker; cook on high pressure: Add onions, butter, 2 teaspoons salt, and ½ teaspoon pepper to the bowl of a multicooker; stir to combine. Secure the lid and adjust vent to seal. Cook on high "Pressure Cook" setting for 5 minutes.
Release pressure and remove lid: Once cycle is complete, immediately release pressure. Carefully remove lid.
Strain onions; return to multicooker: Strain onions and discard excess liquid. Return onions to multicooker.
Cook on sauté setting until caramelized: Cook over high "Saute" setting, stirring constantly, until onions are deeply golden and caramelized, 3 to 5 minutes.
