Preheat the oven to 375°F (190°C) and lightly grease a 9×13-inch casserole dish. In a large skillet over medium heat, cook the ground beef for 7–10 minutes, breaking it apart as it browns. Add the diced onion, bell pepper, and minced garlic. Sauté for about 5 minutes, until the vegetables are softened and fragrant. In a bowl, whisk together the cream of mushroom soup and sour cream until smooth. Season with salt and pepper to taste. Peel and thinly slice the potatoes. Layer them in the bottom of the prepared casserole dish in two even layers, seasoning each layer with a bit of salt and pepper. Spread the cooked beef and veggie mixture evenly over the potatoes. Pour the sour cream and mushroom mixture over the beef, using a spatula to spread it evenly. Top with shredded cheddar cheese and bake uncovered for 30–35 minutes, or until the potatoes are fork-tender and the top is golden. Remove from the oven, sprinkle French fried onions on top, and return to the oven for another 5 minutes, just until the onions are crispy. Let cool slightly before serving. Garnish with green onions or parsley, if using.