In a large pot, over medium heat, add the butter and the oil. Once the butter is melted add the onions and garlic and cook for 3 to 4 minutes until slightly tender. Mix in the flour and cook until the flour is a very pale golden color, about 2 to 3 minutes. Add the potatoes and thyme. Stir in the milk and the juice from the canned clams. Reduce the heat and simmer until the potatoes are cooked through, about 10 minutes.
Remove 2 cups of the chowder and puree in a blender* until smooth. Add pureed chowder, clams and corn to the pot. Season with salt and pepper, to taste, and let simmer for another 5 minutes. Transfer to individual soup bowls or a large soup bowl and serve.
When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.