Set the instant pot on the sauté function and melt the 2 tbsp. of butter with olive oil.
Add thinly sliced onions and stir every few minutes for about 10 minutes before adding salt.
Continue to sauté onions stirring frequently as needed about 15-20 more minutes till they start to brown and caramelize.
Drizzle with balsamic vinegar the last minute of cooking.
Turn off Instant Pot, add remaining ingredients and stir.
Cover with lid. I recommend using a tempered glass lid that you can get for the instant pot. You can however use the lid that came with the instant pot and set to vent for slow cooking.
Set Instant Pot to slow cooker function. Adjust to high setting using the + button until you see the more button is on.
Then set timer for 6 hours and 30 minutes.
Remove bay leaf, serve immediately or add croutons and cheesy topping and melt under broiler.