Cook the cabbage and onion in butter over medium-high until softened. A large wok works best. Season the cabbage with salt and pepper as you work (if it’s bland now, it’ll be bland later!). The wilting process takes about 20 minutes. You may need to reduce the heat as you go to prevent burning. Stir occasionally.
Set the hot mixture aside to cool. Meanwhile preheat the oven to 375F and grease an 11×9-inch baking dish.
Once the filling has cooled to luke-warm, stir in the chopped hard-boiled eggs and the raw egg to bind.
For the batter:
Add the eggs, sour cream, and mayonnaise to a medium bowl. Stir until smooth, then add the dry ingredients.
To assemble and bake:
Spread a little less than half the batter evenly over the bottom of the prepared baking dish. Spoon on the filling.
Top the casserole with the remaining batter, adding it in dollops evenly across the surface and smoothing it with the back of a spoon or spatula to cover the entire surface.
Bake for 45 minutes, until the crust is golden brown and shiny. Let cool at least 15 minutes before slicing so that it holds its shape.
Serve room temperature or warm. It’d be lovely with a bit of sour cream on the side.