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 reserved 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.
When shopping for bell peppers to stuff, choose those with broad bases that will allow the peppers to stand upright in the pan on their own.