Chopped fresh herbs: cilantroparsley, chives, etc.
Slice bagel in half. If the hole is very small, tear out some of the center to make it larger or use a round cookie cutter, small juice glass, or biscuit cutter to cut a larger hole. Lightly spread the cut sides of the bagel with butter. In a small bowl, mash the avocado with the salt, black pepper, and red pepper flakes (if using). Taste and add additional seasoning if desired.
For each bagel half: Heat a non-stick skillet over medium. Place the bagel in the skillet butter-side down. Let toast a few minutes until golden, then flip and place on a plate, toasted side up. Spread the cut side with mashed avocado. Return the bagel to the skillet, avocado-side up. Crack an egg in the center of the hole (for easier transfer, you can also crack the egg in a small bowl, then gently pour it into the hole), then cover the skillet and let cook until the egg is set, about 3 minutes for a medium-soft yolks (if your skillet does not have a lid, you can also place a sheet pan over the top—the idea is to trap heat to cook the egg more quickly). Transfer to a serving plate, sprinkle with any desired toppings. Enjoy immediately.