Heat the oil and butter in a large pan over low heat, and add the diced onion. Caramelize the onions by cooking them on low heat slowly for at least 30 minutes (but ideally 45 minutes or more), stirring often to ensure they caramelize and don’t burn! I often recommend doing this step ahead of time and batch cooking onions for different recipes to save time.

When the onion starts turning golden brown and almost jammy in texture, you can add in the minced garlic. Cook for another minute or two until it turns fragrant.

Add the plain flour and dried rosemary, and cook it off in the onions for a minute or two.

Add the balsamic vinegar and stir into the onions, using your spoon to loosen any bits stuck to the bottom of the pan.

Add the butter beans and stock, stir everything together, and leave to simmer for 10 minutes. The sauce will thicken and be a medium caramel-brown color. Taste and season with salt and pepper.

Pour in the cream and simmer for 5 minutes.