Cut kernels off 8 cobs and set aside. Chop 1 onion.
In a large pot, add corn cobs, 1 chopped onion, 8 cups water, 2 cups heavy cream, 1 teaspoon thyme, and salt & pepper. Combine well and bring to a boil. Reduce heat and simmer 30 to 45 minutes. Note: While stock is simmering, dice remaining 2 onions.
Strain mixture reserving the liquid and discard cobs and onions.
In a large pot, melt ½ cup butter. Add corn kernels and diced onions. Cook over medium heat 10 to 15 minutes. Stirring frequently.
Add reserved stock and bring to a boil. Lower heat and simmer for about 30 minutes.
Using a stick blender or blender puree soup until smooth.