Preheat oven to 350 degrees. Grease 9x13-inch baking pan.
Combine the oatmeal and the boiling water and let the oats soak while you work on the rest of the recipe.
In a medium bowl, whisk the flour, baking soda, salt, ground cinnamon, and ground nutmeg.
Beat the butter, brown sugar, and granulated sugar in a large bowl using a handheld mixer on medium speed until blended. Add the eggs, maple syrup, and vanilla extract and stir to combine using a spoon.
Add the flour mixture in several increments, mixing just until combined.
Stir the softened oats into the mixture. Pour the mixture into the prepared baking dish. Bake for 35 minutes or until a toothpick inserted in the top comes out clean or with a few moist crumbs.
When the cake is almost done baking, stir the melted butter, brown sugar, milk, shredded coconut, and walnuts in a medium bowl.
Spoon the mixture over the warm baked cake, Use an offset spatula to level it. Place it under the broiler for 1-2 minutes. Keep a close eye on it as broilers are unpredicatable.