Simple answer. You should have cut slits in the skins of the apples as steam escape vents. And there wasn't enough moisture in your recipe. Sultanas (grapes) are okay, but you should have used butter inside each apple. There should also have been LIQUID IN THE BOTTOM OF YOUR PAN as well, sometimes flavoured with cider, cranberry juice, vanilla, cinnamon, etc. Some people spoon the liquid over top of the apples as they cook. And keep in mind SOME APPLES (LIKE POTATOES) BAKE BETTER THAN OTHERS. Best baking apples are Granny Smith (for pies) or Rome, Braeburn, Jonathan or Macintosh for whole apples. If the apples were not cooked all the way through, then they weren't left in the oven long enough. But based on your description, they would have burned to crispy critter hockeypucks before the time had passed ;-) Try this recipe instead:
WORLD'S BEST OLD-FASHIONED BAKED APPLES
These apples perfume the kitchen during baking and the juices result in a sweetly spiced sauce. Add a scoop of ice cream for dessert or serve the apples plain for breakfast.
6 large (7 to 10 ounces each) baking apples, such as Braeburn, Empire, Honeycrisp, Jonathan or Rome
1/2 cup apple juice, apple cider or cranberry juice cocktail
2 teaspoons fresh lemon juice (prevents discolouring)
1/2 teaspoon vanilla extract
1/4 cup raisins (or sultanas optional)
6 tablespoons packed light brown sugar
1 tablespoon honey, maple syrup or dark corn syrup
1 1/2 tablespoons unsalted butter, at room temperature
1/4 teaspoon ground cinnamon
Preheat the oven to 350F degrees. Adjust the oven rack to the middle position. Using an apple corer or a small, sharp paring knife, core the apples. (The center cavity should be no more than 1 inch in diameter.) If necessary, trim a thin slice from the bottom of each apple so it stands upright. Using a vegetable peeler or a sharp knife, peel away a 1-inch ring of skin from around the top of each apple. Using the tip of a paring knife, make four small slashes about 1/2 inch deep around the equator of each apple. (These steps aid the release of steam, which helps the apples remain intact rather than collapsing or splitting.) Place the apples in a baking dish just large enough to contain the apples, such as a 9x9" or 7x11" baking dish.
In a bowl, combine the juice or cider, lemon juice and vanilla and pour the mixture around the apples. If desired, divide the raisins evenly among the apples, sprinkling them in the cavity. In the same bowl as you used for the juice mixture, combine the sugar, honey or syrup, butter and cinnamon. Divide the mixture evenly among the apples, spooning it into the cavity.
Bake the apples, basting occasionally with the pan juices, until they are tender when pricked with the tines of a fork, 45 to 65 minutes, depending on the size and variety of apple. Set the apples aside to cool for at least 15 minutes before serving. (May cover and refrigerate for up to several days and reheat until warm, but not hot, before serving.) To serve, spoon the apples into individual bowls and spoon the warm juices over and around the apples.
Makes 6 servings.