Preheat oven to 350 degrees F.
Bring a medium pot of water to boiling.
Add kale and cook for 1 minute.
Drain and rinse with cold water.
In a medium skillet, add pancetta, cooking over medium-high heat until crispy, about 5-7 minutes.
Remove pancetta but leave the grease it released. If it didn't release much grease, add about 1 tablespoon of butter.
Cook onion in pancetta/butter until softened, about 5-7 minutes.
Add garlic and cook another minute.
Add flour, nutmeg, and salt and stir well to combine. Cook about 1 minute, stirring constantly, without letting it brown.
Very slowly, add in the milk, whisking constantly.
Bring to a simmer and let simmer for about 5 minutes, until it has thickened.
Add cheese and crispy pancetta to cream mixture, stirring to combine.
Add kale to greased 8x8" casserole dish.
Pour creamy mixture over kale.
To make the crispy topping, combine melted butter, breadcrumbs, and Parmesan cheese in a small bowl until combined.
Evenly distribute topping over creamy kale mixture.
Bake in preheated oven for 30-35 minutes, until the gratin is bubbling and the topping is golden brown.
Serve and enjoy!