This short guide with spoilers explains how can you get the tender meat.
Tips to Get Tender Meat
Once you get the testament, you continue the story as normal and watch them worry about the windmill.
Inside the windmill is an oven you can use to make the tender meat but you need to get the key from Eugen (whatever his name was).
To get this you have to save the boy who goes missing and talk to people at the party afterwards.
Eugen should give you the key during the party.