In a shallow pan, gently heat about 3 inches of water over medium heat and add the vinegar.
Crack your egg into a small bowl or ramekin.
When the water comes up to just under a simmer, swirl a spoon in the pan to create a whirlpool and gently drop the egg into the center of it. Set the timer for 3 minutes.
Meanwhile, you can use a slotted spoon to gently help form the egg back together, by folding the egg white back onto itself. You can also begin toasting your bread.
Use a slotted spoon to remove the egg from the water and place on a double-layer of paper towels. Make sure all excess water is absorbed.
Spread the avocado on the piece of whole-grain toast, sprinkle the avocado with salt and pepper. Then top with the egg, seasoning again with salt, pepper and parsley. Enjoy on its own or with a tomato salad.