Mix together the mashed avocado, lime juice, cumin, and ¼ teaspoon of the salt.
Lightly toast the bagel flats. As soon as they are done, lay a slice or 1 ounce of cheese over each bagel bottom to melt slowly.
Meanwhile, beat together the egg whites, pepper, and remaining ½ teaspoon of salt. Coat a large nonstick skillet with cooking spray and heat it on medium. Add the egg whites, cover, and cook until firm, 2 to 3 minutes. Divide the whites into four pieces.
Place a piece of egg atop the melted cheese on one bagel bottom. Layer on some sliced tomato and kale leaves. Spread the avocado mixture over one bagel flat top and place it on the prepared bagel bottom, compressing the halves slightly. Cut the sandwich in half before serving. Repeat with the remaining sandwiches.
Tip: For perfectly round egg whites, coat the insides of four 3- to 4-inch metal rings or cookie cutters with cooking spray, place the rings directly in the pan, and pour the egg whites into the rings.