Make sure to collect all the necessary ingredients.
Cook bacon in a large skillet over medium-high heat, turning occasionally until evenly browned. Transfer the cooked bacon onto a paper towel-lined plate and crumble when cool enough to handle. Drain the excess grease from the skillet.
Cook onions and mushrooms in the residual oil in the pan until the onions are see-through and the mushrooms are tender - about 5 minutes.
Grease a 9x13-inch casserole dish with 1 tablespoon of butter. Place potatoes into the bottom of the dish; sprinkle with salt, pepper, and garlic salt. Top with crumbled bacon, then add onions and mushrooms.
Combine milk, eggs, and parsley in a bowl, pour over casserole, top with cheddar, and refrigerate for 8 hours or overnight.
When ready to bake, preheat oven to 400 degrees F. Bake in the preheated oven until set, about 1 hour.
Serve hot and enjoy!