Toast the bread slices in a toaster or on a skillet over medium heat until golden and crisp. Set aside.
Cut the avocado in half, remove the pit, and scoop the flesh into a small bowl. Mash it with a fork until smooth or slightly chunky, depending on your preference. Season with a pinch of salt and black pepper.
Heat the olive oil or butter in a non-stick skillet over medium heat. Crack the eggs into the skillet and cook to your liking—sunny-side up (2-3 minutes), over-easy (flip after 1-2 minutes), or scrambled (stir until set, about 2-3 minutes). Season with salt and pepper.
Spread a generous layer of mashed avocado over each toasted bread slice.
Place one cooked egg on top of each avocado-covered toast. Sprinkle with red pepper flakes if desired, and garnish with fresh herbs or halved cherry tomatoes for extra flavor and color.
Serve immediately while the egg is warm and the toast is crisp. Enjoy your avocado toast with egg!