Preheat the oven to the desired temperature for heating the taco shells.
Heat the olive oil in a large pan over medium heat. Add the onions and a pinch of salt.Cook until soft, stirring frequently, about 5 minutes. Add the ground beef, breaking it up with a spoon or potato masher so it crumbles into small pieces. Cook, continuing to stir until browned, 3-5 minutes. Stir in the taco seasoning, then taste, adding more seasoning or salt if necessary. Remove from the heat.
To make the guacamole, place the avocados in a bowl. Add a generous sprinkling of kosher salt. (The salt is a great abrasive and makes it easier to mash the avocado.) Using a fork, pastry blender, or a potato masher, mash the guacamole until smooth or to your liking. Stir in a few teaspoons of the lime juice to start, then taste, adding more salt and more lime juice to your taste. Stir in the red onion and chopped cilantro if using.
Right before serving, heat the taco shells in the oven, watching carefully as they brown quickly.
When ready to serve, scoop a generous portion of ground beef into each taco shell and sprinkle with cheese and other toppings. Top with guacamole and repeat until they disappear!