Preheat oven to 450 degrees. Melt butter or oil in large cast iron skillet set over medium-high heat.
Reduce heat to medium-low and add onions. Season liberally with salt and pepper, then cover the skillet. Cook, stirring occasionally, until onions are very soft and beginning to caramelize, 10-15 minutes.
Combine pork rindss and cheeses, and sprinkle half of this mixture over onions. Use back of spoon to make 6 little nests in the mixture, and crack 1 egg into each.
Season with salt and pepper, and distribute remaining bread crumbs and cheese mixture over top.
Bake for 5 minutes, or until eggs are barely set. Turn broiler to high and brown top for about 1 minute. Sprinkle with additional cheese, if desired, then serve hot.