Preheat the Oven:
Preheat the oven to 350°F (175°C) and spray a 9 x 13 x 2-inch baking dish with cooking spray.

Cook the Ground Beef:
Add the ground beef to a large skillet and cook over medium-high heat until no pink remains, breaking it into pieces as it cooks. Stir in salt and pepper.

Sauté the Vegetables:
Add the diced onion, red bell pepper, and green bell pepper to the skillet. Cook for a few minutes until the onion is translucent and soft. Drain off any fat and turn off the heat.

Prepare the Potatoes:
Meanwhile, peel the potatoes and slice them thinly. Place the potato slices into a large bowl of cold water.

Make the Sauce:
In a medium bowl, whisk together the evaporated milk, cream of mushroom soup, and cream of chicken soup.

Layer the Casserole:
Drain the potatoes and pat them dry with clean paper towels. Layer half of the potatoes in the bottom of the prepared baking dish. Top with half of the meat mixture, followed by half of the sauce, and then 1 1/2 cups of shredded cheddar cheese. Repeat the layers with the remaining potatoes, meat mixture, sauce, and 1 1/2 cups of cheese.

Top with Fried Onions:
Sprinkle the fried onions on top of the casserole.

Bake the Casserole:
Cover the baking dish tightly with aluminum foil. Bake for 75-80 minutes until the potatoes are tender and the internal temperature reaches 165°F (74°C).

Broil for a Golden Top:
Remove the foil and broil the casserole under high heat for 1-2 minutes until golden brown on top.

Rest and Serve:
Let the casserole sit for about 10 minutes before serving to allow the potatoes to soak up any residual liquid.