Cook the rice as the label directs. Meanwhile, cut the hens in half with kitchen shears, cutting along one side of the breastbone; season with salt and pepper. Heat the canola oil in a large Dutch oven or pot over high heat. Add the hens, skin-side down, and cook until browned, about 5 minutes. Remove and set aside.
Pour off all but about 2 tablespoons oil from the pot. Add the onion and garlic; cook, stirring, until the onion is translucent, about 5 minutes. Return the hens to the pot, skin-side up; add 2 cups water, the soy sauce, sugar, peppercorns and bay leaves. Bring to a boil. Lay the scallions on top of the hens; cover and cook until the scallions are tender and the hens are cooked through, about 10 minutes.
Uncover the pot and add the vinegar (do not stir). Cook, uncovered, until the liquid is reduced by about two-thirds, 8 to 10 minutes. Remove the hens and scallions; discard the bay leaves from the pan sauce. Serve with the rice.