Toast the bread and let it cool (avocado will melt if put directly on hot toast).
Cut the avocado in half and remove the seed. Use a spoon to scoop out the avocado half in one piece and discard the skin. Place cut-side down on a cutting board, and thinly slice the avocado*. Use your hand to press or push the avocado to one side, laying it down. Use a knife to slide under the avocado and transfer it to the toast in one piece.
Drizzle the toast with lemon juice, then sprinkle with everything bagel seasoning, red pepper flakes and flaky sea salt. Serve immediately.
Notes
*You can also mash the avocado in a small bowl or directly on the toast if you prefer a mashed avocado toast.