Toast the sourdough bread slices until deeply golden and crisp, 3–5 minutes.
Halve, pit, and scoop the avocado into a bowl. Add the lemon juice and kosher salt, then mash with a fork until mostly smooth with some small chunks, 30–60 seconds.
Spread the avocado mixture evenly over the hot toast.
Finish with black pepper and red pepper flakes, then drizzle the extra-virgin olive oil over the top.