Preheat oven to 400°Grease a 9 x 13-inch baking dish with cooking spray; set aside.

Melt butter in a large sauté pan over medium-high heat. Add onion, and sauté for 4-5 minutes until soft and translucent. Add garlic and sauté for an additional 1-2 minutes until fragrant. Stir in the flour until it is evenly combined, and cook for 1 more minute. Gradually pour in the stock, and whisk until combined. Add in the milk, salt, pepper, and whisk until combined.

Continue cooking for an additional 1-2 minutes until the sauce just barely begins to simmer around the edges of the pan (avoid letting it come to a boiand thickens. Then remove from heat and set aside.

Spread half of the sliced potatoes in an even layer on the bottom of the pan . Top evenly with half of the sauce.

Then sprinkle evenly with 1 cup of the shredded cheddar cheese, and all of the Parmesan cheese. Top evenly with the remaining sliced potatoes, the other half of the sauce. add the Bacon.

Cover the pan with aluminum foil and bake for 30 minutes. Then remove the foil and bake uncovered for 25-30 minutes, or until the potatoes are cooked through.

Transfer the pan to a cooling rack, and sprinkle with scallions. Serve warm.