Use your hands to squeeze as much moisture as possible out of the grated zucchini.
Beat the eggs lightly in a mixing bowl. Add the ground beef, oats, Worcestershire sauce, 1 1/4 teaspoons salt, all the black pepper, the onion-bell pepper mixture and the zucchini, mixing with your hands until just incorporated. Transfer the mixture to a 9- by 13-inch baking dish, then shape it into a loaf about 5 inches wide and 2 inches tall.
Stir together the molasses, mustard and the remaining 1/4 teaspoon salt and the tomato sauce in a large liquid measuring cup. Pour half to two-thirds of the sauce over the meatloaf, enough so the sauce covers the top of the loaf and drips down the sides a little, but without much overflowing onto the pan. Reserve the rest of this ketchup-like sauce to serve alongside the finished meatloaf.
Arrange the thinly sliced onion rings on top. Bake on the middle rack for about 1 hour, or until an instant-read thermometer inserted into the center registers 160 degrees. Let the meatloaf rest for 5 to 10 minutes before slicing.
Nutrition information per serving: