Quick answer: For the Garfield: Escape from Monday Odie's antidote barbecue objective, find the meat chops still around Odie's ranch. Spray them with Liz's antidote, then toss them onto the barbecue so Odie eats them. Source
Players also call it Odie ranch meat chops or Odie barbecue antidote.
How to give Odie the antidote
Find the meat chops
Look for the meat chops still around Odie's ranch. Source
Spray the chops
Spray the meat chops with Liz's antidote. Source
Use the barbecue
Toss the sprayed chops onto the barbecue so Odie eats them. Source

Do not stop after spraying the chops: Liz's instruction also says to toss them onto the barbecue so Odie eats them. Source
FAQ
Related
- Where to Find Odie's Antidote at the Clinic covers where to find the antidote.
- How to Power Squeak's Barn Portal covers Squeak's separate portal objective.
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