Preheat oven to 200°F. Move onion slices to edges of grill. Sprinkle pork chops with a total of 1/4 teaspoon salt and 1/8 teaspoon pepper. Place as many pork chops as will fit in the center of the grill. Arrange half the oregano over pork chops (save 2 sprigs for garnish). Close grill and cook through, 3 to 4 minutes. Transfer to a rimmed baking sheet. Place in oven to keep warm. Repeat with remaining pork chops.
Sprinkle tomatoes with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Lightly coat tomatoes with cooking spray; place on grill. Arrange remaining oregano over tomatoes. Turn onions over. Close grill and cook tomatoes until hot and softened, 1 to 2 minutes.
Transfer tomatoes and onions to a warm platter. Arrange pork chops on top. Pour juices from baking sheet over pork chops; garnish with oregano. Serve with your favorite wild-rice recipe.
Alternate preparation: Pork chops can also be prepared on an outdoor grill or stovetop grill pan. Cook onions over medium heat until tender, about 10 minutes. Meanwhile, grill pork chops 5 to 6 minutes; turn once halfway through cooking. Grill tomatoes about 2 minutes.