Scoop avocado into a small mixing bowl and mash with a fork. Add lemon juice, onion, salt, and pepper. Mix well.
Divide the mixture and spread on toast.
Garnish with tomato, and cilantro or arugula. and serve immediately.
Get creative! Other toppings to consider are 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 or hot sauce.