There are 2 ways of getting the Lava Cookie item in FR/LG.
The first one is very simple - to buy a Lava Cookie in the Market Stall on Two Island for 200 Pokémon Dollars. This is only possible after getting access to One, Two and Three Island.
The second one is very complicated.
This is the procedure for getting one Lava Cookie earlier in Vermilion City:
- After boarding onto the S.S. Anne for the first time, you have to acquire HM01, Cut. Alternate methods are trading in a Pokemon knowing Cut into your game (legitimate) or hacking in the HM Cut (illegitimate); these do not require boarding S.S. Anne.
- If you board the S.S. Anne for acquiring Cut, you must not trigger the end scene of the S.S. Anne departure. This can be accomplished by losing to a trainer on the ship after receiving Cut from the ship captain, whiting out and waking up in Vermilion City's Pokemon Center.
- After waking up in the Pokemon Center, do not board S.S. Anne and continue with your journey until you get the Surf HM in Fuschia City's Safari Zone.
- After acquiring Surf, teach it to a Pokemon in your party and go to the S.S. Anne. After flashing the S.S. Anne ticket to the guard, you will step on a platform leading to S.S. Anne.
- Instead of boarding the ship, step right and surf in the water to the right. You will then reach a platform with a truck on it.
- Go to the bottom of the platform, and a hidden Lava Cookie is present on the second-last middle tile. Face in the direction of the tile and click "A" to receive the Lava Cookie.
- Go back to the S.S. Anne platform and continue with your journey. You may or may not board S.S. Anne.
Side note: If you board S.S. Anne without having a Pokemon knowing Surf in your party after waking up in the Pokemon Center, you will not be able to receive the Lava Cookie. The game will automatically make you step all the way to the ship and watch it departure, and then send you out of the harbour with no way of getting in again, thus not being able to use Surf in the water to the right. (Here's the video of that happening)
Source 1, Source 2
Clearly, there is no hidden Mew beneath the truck on the platform, so using Strength on it or anything else won't do any good. The Lava Cookie is also not underneath the truck but instead to the south of the platform.
Hope this helps!
