Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch casserole dish with cooking spray or butter.

In a large skillet, cook the lean ground beef over medium heat, breaking it up with a spatula as it cooks, for about 7-10 minutes until browned and a food thermometer reads 160°F. Add the diced onion, diced green bell pepper and minced garlic. Sauté for about 5 minutes until the vegetables are soft and fragrant.

In a bowl, combine the condensed cream of mushroom soup and sour cream. Mix until smooth, then season with salt and pepper to taste.

Peel and thinly slice the Russet potatoes. Arrange the slices in two layers in the bottom of the greased baking dish, seasoning each layer lightly with salt and pepper. Spoon the cooked ground beef and vegetable mixture evenly over the potato layers. Pour the cream sauce over the beef and veggies, spreading it evenly with a spatula.

Sprinkle the shredded cheddar cheese evenly over the top of the casserole. Bake in the preheated oven for 30-35 minutes, or until the potatoes are tender and the top is golden brown.

Remove the casserole from the oven and sprinkle French fried onions over the top. Return to the oven for an additional 5 minutes until the onions are crispy and the center reaches 165°F on a food thermometer. Let the casserole cool for a few minutes before serving. Garnish with sliced green onions or chopped fresh parsley, if desired.