Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until it's fully browned. Drain any excess fat.
Step 3: Add the chopped onion to the skillet with the beef and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Step 4: Stir in the diced tomatoes (undrained), tomato sauce, Italian seasoning, salt, and pepper. Bring the mixture to a simmer, then reduce heat and let it cook for 5 minutes, allowing the flavors to meld.
Step 5: Remove the skillet from the heat. Fold in the cooked white rice and chopped bell peppers.
Step 6: Transfer half of the beef and rice mixture to the prepared baking dish. Sprinkle with half of the shredded cheddar cheese.
Step 7: Top with the remaining beef and rice mixture, spreading it evenly.
Step 8: Cover the baking dish with aluminum foil and bake for 25 minutes.
Step 9: Remove the foil, sprinkle the remaining cheddar cheese over the top, and bake uncovered for another 10 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Step 10: Let the casserole rest for 5-10 minutes before serving. Garnish with fresh parsley, if desired.