How to Make It
Preheat oven to 425°F. Generously grease a 9 1/2-inch pie plate with 1/2 Tbsp. butter. Sprinkle panko breadcrumbs into the pie plate, and turn the pie plate so the panko evenly coats the inside of the dish.
Melt the remaining 1 Tbsp. butter in a skillet over medium-high heat. Add onion, pancetta, and salt, and cook until the onion is slightly translucent, about 7 minutes. Remove from pan and spread evenly into pie plate.
Scatter Swiss cheese and Gruyère cheese evenly on top of the onion mixture.
In a large mixing bowl, whisk together the eggs and half-and-half until completely combined. Gently pour into pie plate.
Bake in preheated oven until the center does not move when the pie plate is lightly shaken, about 35 minutes. Allow to cool before slicing.