Toast the sourdough bread until golden and crisp. You can use a toaster or toast it in a dry skillet over medium heat for about 2–3 minutes per side.
Cut the avocado in half, remove the pit, and scoop the flesh into a small bowl.
Add lemon juice, kosher salt, and black pepper to the avocado. Use a fork to mash everything together until it's as smooth or chunky as you like.
Spoon the mashed avocado evenly over the toasted sourdough slices, spreading it out to the edges.
Sprinkle the everything seasoning and red pepper flakes evenly over the top.
Drizzle a small amount of extra-virgin olive oil over each slice just before serving.
Serve immediately while the toast is still warm.