While countering your enemy and having a grenade/flash grenade equipped, you will feed it to an enemy. The enemy will stagger for a while with the grenade in their mouth. Simply shoot the grenade to get the "Bon Appétit" achievement. If you do not get it from natural gameplay on your first playthrough, it can quickly be done at the start of the "B" scenario (2nd run) for both Leon and Claire. In the first room you enter is a grenade, only a few seconds after starting the "B" scenario story.