Adjust an oven rack to the middle position and preheat oven to 350 degrees. In a large pot over high heat, bring 4 quarts water and 1 tablespoon salt to boil. Add the bell peppers and cook until they begin to soften, about 3 minutes.
Remove the peppers from the water, allowing any excess water to drain back into the pot. Place the peppers, cut-side down, on a paper-towel lined plate.
Return the water to a boil. Add the rice and cook uncovered until tender, about 13-15 minutes. Drain thoroughly and transfer to a large bowl.
Meanwhile, in a 12-inch skillet over medium-high heat, heat oil until shimmering. Add onion and cook until softened, about 5 minutes.
Stir in beef and cook until no longer pink, about 5 minutes, breaking up clumps with a wooden spoon. Stir in the garlic until fragrant, about 30 seconds.
Transfer beef mixture to the bowl with rice. Add the tomatoes and their juice, ketchup, parsley, and 1 cup of the cheese and stir to combine. Season to taste with salt and pepper (I like 1/2 teaspoon salt and 1/4 teaspoon pepper).
Place the peppers, cut-side up, in a 9 by 9-inch baking dish. Divide the filling evenly among the peppers. Sprinkle them with the remaining 1/4 cup cheese. Bake until the cheese is browned and the filling is heated through, about 30 minutes.