Preheat oven to 180°F.

Pour the heavy cream into the large oven-safe dish. The dish should be large enough that the cream is only 1/2"-1" deep.

Place the dish in the oven and bake for 12 hours. 

Remove dish from the oven and allow to cool to room temperature. A thick skin will have formed over the top while baking.

After the dish has cooled, place it in the refrigerator for 12 hours. 

Scrap the thick skin off the top of the cream and place the cream in a large bowl. Discard any extra liquid. 

Mix thoroughly until the cream thickens.