Scoop the avocado into a small mixing bowl. Add lemon juice and onion. Mix and mash the avocado with the back of a fork until it is as lumpy or smooth as you wish. Season to taste with salt and pepper.
Lightly brush or drizzle olive oil on the top side of your toast. Divide the avocado mixture and spread evenly.
Top with chopped tomato, fresh salsa, and/or cilantro and serve right away.
Get creative! Here are some toppings to consider: fresh basil leaves, red pepper flakes, roasted sunflower seeds, minced sun-dried tomatoes, strips of vegan or dairy cheese, coarsely chopped artichoke hearts, a drizzle of balsamic vinegar.