Bulgogi sauce: In a small saucepan, whisk together soy sauce, brown sugar, garlic, sesame oil, ginger, and pear juice. Warm over low heat until the sugar melts. allow to cool and keep in a small container in the fridge.
Gochujang aioli: Combine everything in a bowl and set it aside.
Preheat the oven to 375F. Lightly grease an 8x6-inch baking dish with cooking spray. Set aside.
In a large skillet, heat oil over medium-high heat until hot. Add onion and cook for 5 to 7 minutes or until tender, stirring frequently. You can also cook onions a little bit longer to bring more sweetness. Add garlic, and cook for another 2 minutes.
Brown the ground beef, drain excess fat if any, and add 1.5 cups of bulgogi sauce. Season with black pepper to taste.
Transfer ground beef mixture to the prepared baking dish and spread evenly, then the cheese, and finally, line up the crispy potato rounds. Bake for 45 minutes or until the potatoes are golden brown and cheese is melted.
Sprinkle with scallions and serve with gochujang aioli.