How to Make It
Set Instant Pot to sauté. Add butter and 1 tablespoon of the olive oil; heat until butter is melted. Add leek, carrot, celery, mushrooms, and 1 tablespoon of the garlic; cook, stirring often, until softened, about 5 minutes.
Add cannellini beans, diced tomatoes, vegetable broth, salt, and pepper; cook, stirring often, for 2 minutes. Cover Instant Pot and fasten lid. Lock and seal steam valve. Set to HIGH PRESSURE for 3 minutes. Release pressure naturally. (Steam is fully released when pressure valve drops.) Uncover and turn off Instant Pot. Stir in sherry vinegar.
While cassoulet is cooking, heat remaining 1 tablespoon olive oil in a small skillet over medium. Add remaining 1/2 tablespoon garlic, and cook until golden brown, about 30 seconds. Stir in panko, and cook, stirring often, until golden brown, about 2 minutes. Stir in parsley, thyme, and rosemary. Sprinkle panko mixture over cassoulet and serve immediately.