Preheat the oven to 190c, gas mark 5.
Slice the tops off the onions, and leave on the skins.
Place the onions in a square baking dish.
Drizzle the olive oil over the onions, and place about a third of the butter on top of each one.
Season generously with salt and pepper, and top with a rosemary sprig.
Bake for 25 minutes, or until the onions are starting to go soft.
Increase the oven to 200c, gas mark 6 and cook for another 15 to 20 minutes, until the onions are completely tender and the tops are browned.
Remove from the oven, take off the skins, slice and serve.
Serves: 3 as a side
Preparation: 10 mins
Cooking: 45 mins
3 Albert Bartlett onions
3 or 4 tbsp of olive oil
30gĀ unsalted butter
Salt and pepper
Fresh rosemary sprigs