Preheat the oven to 350. Cut off both ends of the onion. Peel off the outer layer or more of the onion until you are to the tender center. If taller than 2 inches, cut into 1-inch slices. Spread out with the wider side up in a baking dish.

Half of the balsamic & olive oil will be used when the beans are added, so start by reserving half of each. Drizzle a splash of the balsamic, dividing it across each onion slice and giving the onion a chance to suck up the vinegar. Afterwards, drizzle on the olive oil in a similar fashion. Sprinkle each slice with salt and pepper.

Place in the oven to roast. Turn over every 15 minutes to keep the slices coated and moist. Bake for about 35-45 minutes, until the onions are just barely tender. Toss in the white beans, sprinkle on more salt if the beans are unsalted, a pinch more pepper, and the remaining olive oil, balsamic and all of the fresh minced rosemary. Bake for another 5-10 minutes until the beans are just heated through and incorporated. Serve while hot.