Toast the bread to your desired crispiness.
Mash the avocado in a small bowl and season with salt, pepper, and lemon or lime juice (optional).
Bring a small saucepan of water to a gentle simmer. Add white vinegar.
Create a whirlpool in the water and carefully slip in the cracked egg.
Cook for 2-3 minutes, or until the whites are set but the yolk is still runny.
Remove the egg with a slotted spoon and drain on a paper towel.
Spread the mashed avocado evenly over the toasted bread.
Top with the poached egg and sprinkle generously with everything bagel seasoning.
Season with red pepper flakes if desired. Serve immediately.