Preheat oven to 400°F. Spray 8-inch dish with nonstick spray.
Layer half potatoes in dish. Season with salt and pepper. Add onions and then add remaining potatoes. Season with salt and pepper.
Melt butter in saucepan on medium heat. Whisk in flour, whisking constantly for 1 minute. Gradually add milk (about ¼ cup at a time), whisking well after each addition.
Cook 3-5 minutes, whisking constantly, until mixture thickens.
Remove from heat. Stir in all cheese. Continue stirring until all cheese has melted.
Add French Onion dip and stir until combined.
Pour mixture over potatoes and cover with aluminum foil that has been sprayed with nonstick spray.
Bake until potatoes are tender and sauce is bubbly (about 45 minutes). Uncover for last 10 minutes to allow potatoes to brown.
Let cool 15 minutes over a wire rack. Garnish as desired and serve.