Dice the apples, and toss with the lemon juice to prevent browning. Set aside.
Bring the cider up to a boil in a large pot, then add the oats. Stir constantly until it begins to thicken, then lower the heat to maintain a simmer.
Stir occasionally during the simmering phase, cooking for 15-20 minutes. Remove oatmeal from heat. It should still be a bit runny, and not quite at a porridge-like consistency.
Combine the cinnamon, nutmeg, and allspice. Add this blend to the oatmeal, reserving some to sprinkle on top. Fold in the diced apples and 1/2 cup of the nuts. Sprinkle with brown sugar.
Pour into a sprayed baking dish, and top with remaining nuts and spices. Bake at 350 degrees for 15-25 minutes, or until the top is golden brown.