Place the oats in a medium bowl. Add the boiling water, mix, and let stand 20 minutes. Preheat the oven to 350 degrees F. Lightly butter a 9-inch cake pan, round or square. In a mixer fitted with a paddle attachment (or using a hand mixer), cream the butter until smooth. Add both sugars and mix until smooth and fluffy. Add the eggs and vanilla and mix just until combined. Add the rolled oats and mix just until combined.
In another bowl, mix the flour, baking soda, cinnamon, nutmeg, and salt. Working in 3 batches, add the dry ingredients to the butter-oat mixture, mixing until just combined after each addition. Pour into the prepared pan. Bake until a tester inserted into the center of the cake comes out dry and almost clean (a few crumbs are okay), 40 to 50 minutes.
When the cake comes out of the oven, preheat the broiler to high. Spread the coconut topping evenly over the cake and broil until golden brown, about 2 minutes, watching carefully to make sure it does not burn. Let cool at least 10 minutes before serving.
In a medium bowl, mix the butter, sugar, and cream. Add the nuts and coconut and mix just until combined.