Pour the oil in the Instant Pot and select “Sauté”. Then add the onion and cook for about 5 minutes.
Add garlic and rosemary and sauté for about 1 minute.
Select “Cancel” and stir in the broth, lemon juice and black pepper.
Place the mussels in steamer trivet and arrange the trivet in Instant Pot.
Next, secure the lid and cook under “Manual” and “Low Pressure” for about 1 minute.
Select the “Cancel” and carefully do a quick release.
Remove the lid and transfer the mussels in serving bowl and top with the cooking liquid from the pot.
Top with sour cream and serve hot.