This page of the Resident Evil Village guide explains where to find Luiza's Necklace (one of the unique treasures from the golden chests), how to get Luiza's Key and how to use it.
The earliest you can try to find Luiza's Necklace is after returning to the village from Castle Dimitrescu. Go once again to Luiza's House, adjacent to Fallow Plot.
The debris is all that's left of the burned house, but you'll find an interactive chest in front of it.
You can take the necklace out of the box.
Look at the necklace in your inventory (Key Items tab) to take out Luiza's Key.
You must then reach the chest shown in the above images. It is in one of the side areas of the Lone Road location, which is situated to the west of the Altar. You can use Luiza's key to open the chest. Inside you will find the Cesare's Goblet treasure.