The night before you make the baked oatmeal, put the nuts, oats, lemon juice, and sea salt in a large mixing bowl and fill with water to cover by an inch. Give it a stir to combine and leave the bowl in a warm spot in your kitchen over night. This soaking process helps the oats and nuts digest better, and makes more of their nutrients available to absorb.
The next morning put the oat mixture in a strainer to drain.
While the oat mixture drains, put the rest of the ingredients except the sliced apples for the top into the large mixing bowl, and combine it with the drained oat mixture.
Pour the batter into a buttered 9x13 baking dish, top with sliced apples, sprinkle with cinnamon, and bake uncovered at 375 degrees for 1 hour. Let the baked oatmeal cool about 10 minutes before cutting.