Add roughly chopped cabbage and diced onion to Instant Pot.
Add cold butter, kosher salt, and freshly ground black pepper, then pour in chicken broth. Quickly stir once or twice, then secure Instant Pot lid on pot with vent in Sealing position.
Set Instant Pot to Manual High Pressure for 7 minutes. Allow 5 to 10 minutes for Instant Pot to come to pressure before cook time begins. Once 7-minute cook time is up, carefully Quick Release pressure and remove lid.
Stir cabbage mixture again. Use slotted spoon to remove cabbage from pot and season with additional kosher salt and black pepper if desired. Serve hot.
