Heat oil and butter in a Dutch oven or stock pot over med high heat; chop onions, carrots, and celery and add to pot. Sauté until onions are translucent. While onions, carrots and celery are sautéing peel potatoes and cut into 1 inch cubes. Add to pot and stir. Add chicken broth and bring to a boil. Turn to a simmer and add salt, pepper and thyme to taste. Simmer until potatoes are soft about 30 to 45 minutes. Using an immersion blender, puree soup to desired consistency. Cut Kielbasa into 1 inch pieces,add to the soup, and heat through.
If you don’t have an immersion blender carefully use a regular blender pureeing one ladle of room temperature soup at a time. Be careful not to over fill your blender as warm or hot liquids can exploded through the top causing a mess and possible burns. You can also use a potato masher resulting in a chunkier soup. Return soup to the pot, add kielbasa and heat through.