Heat up your BBQ or grill-pan to medium-high heat and spray with non-stick spray.
Cut avocados in half and remove pits.
Brush inside half of each avocado lightly with olive oil.
Place face down on BBQ or grill-pan for about 5 minutes (BBQ temps vary). Press down occasionally to ensure grill marks are created and heat penetrates.
Using tongs remove from grill and set aside until ready to eat.
For the Grilled Avocado Toast:
Scoop out flesh from avocados and place in a large bowl.
Juice limes into bowl and using the back of a fork or a potato masher, mash up avocados until chunky but spreadable.
Toast bread (if desired) and top with avocado-lime mixture.
Sprinkle with salt and red pepper flakes if using.