Preheat the oven to 400°F. Spray a 9x13-inch casserole dish with nonstick cooking spray and set aside.

Cook the bacon by lining a rimmed baking sheet with aluminum foil and placing a wire rack on top. Lay the bacon slices on the rack and bake for 12 minutes. The bacon will not be crispy, but it will continue cooking with the potatoes. Allow the bacon to cool, then chop into small pieces. Set aside.

In a large mixing bowl, whisk together the mayonnaise, diced onions, garlic, salt, and pepper until thoroughly combined. Add the potatoes, ¾ cup of the shredded cheese, and about ¾ of the chopped bacon. Stir together with a spatula until the potatoes are fully coated in the mayonnaise mixture.

Pour the potato mixture into the prepared baking dish and spread into an even layer. Sprinkle the remaining ¼ cup shredded cheese over the top. Bake until the potatoes are soft and hot throughout, about 1 ½ hours.

Sprinkle the green onions and the remaining chopped bacon on top, along with any other desired toppings, and serve. Store cooled leftovers in an airtight container in the fridge for 3 to 4 days or in individual freezer bags in the freezer for up to 1 month. Reheat individual servings in the microwave in 30-second intervals until warmed through.