Preheat the oven to 350°F and grease a 9x13-inch baking dish.
Prepare the potatoes: Peel and slice them thinly, then soak in cold water for 5-10 minutes to remove excess starch. Drain and pat dry.
Cook the ground beef: In a large skillet over medium-high heat, brown the ground beef. Add the onions and bell peppers and cook until softened, about 5 minutes. Drain excess grease. Season with salt and pepper.
Make the creamy sauce: In a mixing bowl, whisk together the cream of mushroom soup, cream of chicken soup, evaporated milk, and garlic powder until smooth.
Assemble the casserole: Layer half of the potatoes in the bottom of the prepared baking dish. Spread half of the beef mixture over the potatoes, then pour half of the creamy sauce on top. Sprinkle with 1 cup of shredded cheese. Repeat with the remaining ingredients.
Bake: Cover tightly with aluminum foil and bake for 75 minutes.
Crisp the topping: Remove the foil, sprinkle the crispy fried onions on top, and broil for 1-2 minutes until golden brown.
Rest before serving: Let the casserole sit for 10 minutes before slicing to allow the layers to set.