How to Make It
Heat oven to 400° F. Whisk together ¼ cup of the ketchup and the brown sugar in a small bowl; set aside.
Combine the oats and milk in a large bowl. Let stand until the oats are soft, 5 minutes. Whisk in the eggs, onion, Worcestershire sauce, thyme, the remaining ¼ cup of ketchup, ¾ teaspoon salt, and ½ teaspoon pepper. Add the beef and pork and gently mix with your hands until just combined.
Transfer to a 9-by-13-inch or other 3-quart baking dish and form into an 8-by-5-inch oval. Brush the top and sides of the meat loaf with the sweetened ketchup. Arrange the tomatoes around the sides.
Bake until an instant-read thermometer inserted in the center of the meat loaf registers 165° F, 40 to 50 minutes. Let rest for 5 minutes before slicing.