Heat the olive oil and butter in a large pot over medium until the butter foams.
1 tbsp extra-virgin olive oil, 2 tbsp unsalted butter
Add the onion and cook, stirring often, until translucent, about 5 minutes.
1 medium onion
Stir in garlic, tomato paste, and mustard; cook, stirring constantly, until the paste darkens, about 1 minute.
2 cloves garlic, 3 tbsp tomato paste, 1 tsp Dijon mustard
Add diced tomatoes, beans, stock, oregano, and red pepper flakes; bring to a simmer.
1 15-ounce can cannellini beans, 1 28-ounce can diced tomatoes, 3 cups low-sodium chicken or vegetable stock, 0.5 tsp dried oregano, 0.25 tsp crushed red pepper flakes
Season with salt and pepper; simmer uncovered until flavors meld, about 15 minutes.
kosher salt and freshly ground black pepper
Remove from heat and let rest for 5 minutes.