Preheat oven to 375 degrees. In a Dutch oven or large stock pot with a tight-fitting lid over medium-high heat, add ground beef, onion, garlic, and 1/2 teaspoon salt.
Cook until browned, about 5 minutes, then drain fat well. Add water, diced tomatoes, and ketchup. Bring to a boil, then stir in rice, bell peppers, parsley, and 1/2 teaspoon salt.
Cover, transfer to the oven, and bake for 15 minutes. Remove cover and season to taste with salt and pepper.
Top with shredded cheese, then return uncovered to the oven. Bake until cheese is melted, about 5 minutes longer. Garnish with fresh parsley if desired.
Notes
Tomatoes: If you'd like to use the juice from the canned tomatoes, drain it into a 2-cup measuring cup, then add water to reach 2 cups to use in step 2.
Yield: This recipe makes 8 cups of Stuffed Pepper Casserole, enough for 4 hearty servings, 2 cups each.
Storage: Store leftovers covered in the refrigerator for up to 4 days.