How to Make It
Preheat oven to 375°F. Peel onions, and trim base of each so that onion sits flat. Using a small paring knife, cut out a hollow (about 1 inch in diameter) from top of onions. Place onions in an 8-inch square baking dish, and add water to a depth of about 1/4 inch up sides of dish.
Stir together butter, miso, and stock base in a small bowl. Spoon miso butter evenly (about 1 tablespoon) into hollow of each onion.
Bake onions in preheated oven until cooked through and can easily be pierced with a paring knife, about 1 hour, 20 minutes, basting with cooking juices every 30 minutes. Transfer onions to a platter; drizzle with about 1/4 cup cooking juices from baking dish, and serve.