In a large sauté pan, heat the oil over medium-high heat. Add the onion and cook, stirring often, until softened, about 3 minutes. Add the beef and cook, stirring often to break up the meat, until no longer pink, about 6 minutes. Add salt and pepper to taste. Stir until well combined. Taste and adjust the seasoning. Set aside.
Preheat oven to 375°. Spray a 9x12 baking dish with olive oil. Using a sharp knife or mandolin slice the potatoes very thin. Create a thin layer of potatoes covering the entire bottom of baking dish.
In a medium bowl combine the milk, cream of chicken/or mushroom soup, sour cream and herbs. Whisk until well blended.
Start by layering the meat, wet ingredients and cheese. Repeat. Ending with a layer of cheese. Cover with foil and bake. Last 15 minutes remove foil and bake until cheese is bubbly. Sprinkle with fresh chives or parsley.