There are many special achievements in The Lake House, and "Seasoning the Soup" is one of them. To unlock it, go to the "Prepare Dinner" level, use the knife on the fly at the cake to get a dead fly. It's not too difficult.
How to Get the "Seasoning the Soup" Achievement in Rusty Lake: The Lake House
Seasoning the Soup
Seasoning the Soup
In the "Prepare Dinner" level, use the knife on the fly at the cake to obtain a dead fly.
Add a dead fly to the soup. Unlock after drinking the soup and completing William's scene.
Related Guide: How to Get the "Check Furniture" Achievement in The Lake House