Preheat the oven to 350ºF.
In a small saucepan, over medium-high heat, add butter and oil.
Once the butter has melted, add onion, garlic and rosemary. Sauté until aromatics are softened, about 3-5 minutes. Add beans and stir a few times to combine. Add salt and pepper to taste.
Transfer to a small baking dish or four small ramekins for individual portions, making sure to remove woody stems of rosemary before baking.
Bake in the oven for 10-15 minutes, until beans have fallen apart slightly and created a creamy consistency. Drizzle the top with a small amount of olive oil for a crispier bite.