Preheat and Prep: Preheat your oven to 350 degrees Fahrenheit and lightly butter a 9x13-inch baking dish. Arrange the sliced potatoes in overlapping layers on the bottom, seasoning each layer generously with salt and pepper.
Create the Custard: In a medium bowl, whisk together the milk, eggs, minced garlic, 3/4 teaspoon salt, and 1/4 teaspoon black pepper until well combined and smooth.
Layer the Cheese and Bacon: Sprinkle half of the cheddar, Monterey Jack, and Gruyere cheese blend over the potato layer, then scatter half of the crumbled bacon on top. Repeat with another layer of the remaining cheese and bacon.
Pour the Custard: Carefully pour the milk and egg mixture evenly over all the layers, making sure it distributes throughout. The custard will soak into the potatoes and create a creamy texture as it bakes.
Bake: Cover the casserole with aluminum foil and bake for 30 to 35 minutes until the potatoes are tender and the custard is mostly set. Remove the foil and bake for an additional 10 to 15 minutes until the top is lightly golden and the edges are bubbly.
Cool Slightly: Let the casserole rest for 5 minutes after removing from the oven so the custard sets completely and slicing becomes cleaner.
Garnish with Parsley: Sprinkle the fresh chopped parsley over the top just before serving for a pop of color and herbaceous freshness.
Serve Warm: Cut into generous squares and serve directly from the baking dish while still warm for the creamiest, most satisfying experience.
