ROAST: Preheat the oven to 400 F / 205 C. Carefully cut the acorn squash in half, then scoop out the seeds. Cover the bottom of a 7 x 11 baking dish with water, then place the acorn squash halves, cut side down, into the dish. Roast until the squash is tender, about 45 minutes. Remove from the oven and allow to cool.
SIMMER: When the cooked squash is cool enough to handle, scoop out 2 cups / 400 g of the flesh and add it to a medium sized saucepan. Add all other ingredients to the pan over medium heat and bring almost to an even simmer, stirring to combine the ingredients evenly. Reduce the heat a little to maintain an gentle simmer and cook, stirring occasionally, until the apple and cranberries have softened and the oatmeal has thickened, about 20 minutes. Serve drizzled with a little maple syrup or raw honey, if you like. I like to finish mine off with some extra chopped apples as a nice crunchy topping, too!