In a small bowl, stir together the oats and milk. Thinly slice 1/4 of the onion and set aside. Finely chop the remaining onion. In a large bowl, combine the turkey, oat mixture, chopped onion, bell pepper, eggs, Worcestershire sauce, ketchup, salt and a few grinds of pepper. Mix just until well combined.
Transfer the mixture to a 9-by-13-inch baking dish and shape into a loaf about 5 inches wide and 2 1/2 inches high. Pour the tomato sauce over the meatloaf and sprinkle with the sliced onions. Bake for about 1 hour or until an instant-read thermometer registers 160 degrees.F.
Remove from the oven and let rest for 10 to 15 minutes before slicing.