Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch pan.

Pour boiling water over oats in a small bowl. Mix well, and set aside to cool.

Beat brown sugar, white sugar, butter, and eggs in a large bowl until creamy. Mix in flour, baking soda, cinnamon, nutmeg, and salt. Stir in cooled oatmeal until combined. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 35 minutes.

While the cake is baking, make the topping: Combine coconut, white sugar, and brown sugar in a mixing bowl. Add melted butter, heavy cream, and vanilla; stir until well combined.

Remove cake from the oven; turn on the broiler.

Drop small spoonfuls of topping mixture over the cake, then carefully spread topping into an even layer to cover the surface.

Place under the preheated broiler until topping is bubbly and lightly browned, 2 to 5 minutes.