Preheat the oven to 400 degrees F. You can also use a toaster.
Fill a small pot/skillet with enough water and bring to a boil, add distilled white vinegar (if you're using it), while the water is boiling, crack each egg into small bowls. When water gets to a boiling point and still boiling turn off the heat and gently drop an egg at a time and cover with a lid, leave for about 5 minutes to poach.
While waiting for the eggs, get the toasted bread out if they're done. In a bowl, add avocado and mash with a fork, add lemon juice, onion, salt, and pepper and mix all together. Feel free to use any other ingredients you like in your avocado mash. Spread the avocado on the toasted bread (one side).
By this time the eggs would've been done, bring them out and dry on paper towels lined on a plate, and place on the toasted bread with avocado mixture.
Sprinkle a pinch of salt and pepper. I wanted a little heat I drizzled some sriracha sauce on top.
Serve and enjoy! I added some sauteed tomatoes on the side.