Heat oven to 375° F. Cook the rice according to the package directions.
Heat the oil in a large skillet over medium-high heat. Add the scallion whites and beef and cook, breaking the beef up with a spoon, until no longer pink, 3 to 5 minutes. Stir in the corn, chilies, cumin, cooked rice, ½ cup of the Monterey Jack, ½ teaspoon salt, and ¼ teaspoon black pepper.
Arrange the bell peppers, cut-side up, in a 9-by-13-inch baking dish or pan. Divide the beef mixture among the bell peppers, add ½ cup water to the dish, tightly cover the dish with foil, and bake until the bell peppers are soft, 30 to 40 minutes. Uncover, sprinkle with the remaining ½ cup of Monterey Jack, and bake until browned, 5 to 7 minutes more.
In a small bowl, whisk together the yogurt and ¼ cup water. Drizzle over the bell peppers and top with the salsa and scallion greens.