Preheat your oven to 375°F (190°C). Generously grease a 9x13 inch baking dish with the softened butter.
Peel and thinly slice the Russet potatoes. Arranging them in even layers ensures consistent cooking.
Place half of the sliced potatoes into the prepared baking dish. Season lightly with salt and pepper.
Sprinkle half of the shredded Cheddar cheese, half of the Monterey Jack cheese, and half of the crumbled beef bacon over the first layer of potatoes.
Layer the remaining potatoes on top of the cheese and beef bacon mixture.
In a medium bowl, whisk together the milk and eggs until fully combined and smooth. Pour this custard mixture evenly over the potatoes.
Cover the dish tightly with aluminum foil to trap moisture.
Bake in the preheated oven for 90 minutes. The potatoes should be tender when pierced with a knife.
Remove the foil during the last 15 minutes if a browned, bubbly top is desired.
Remove from the oven and let the casserole rest for 15 minutes before serving. This allows the custard to set and makes slicing easier.