In a large pot, melt the butter and oil over medium-high heat.
Add some salt and pepper to the pot along with the chopped celery, onion, and carrots. Cook for about 15 minutes or until softened.
Add the garlic powder and stir. Continue cooking for one or two more minutes.
Bring to a boil after adding the stock.
After 15 minutes, turn off the heat and let the mixture simmer.
Pour in the half and half or milk and whisk to mix after allowing it cool.
Use an immersion blender or a stand blender to carefully blend the soup.
Add the fresh parsley and chili powder, and leeks on top after seasoning with salt and pepper to taste.
NOTES
To ensure that the vegetables cook fast and soften, chop them into little pieces.
An immersion blender is the quickest and most straightforward method for blending the soup in the pot. Alternatively, you can spoon it into a stand-alone blender, but be careful—it will be boiling.
Let the soup cool completely before storing if making ahead.
If you're using dried herbs rather than fresh ones, add them to the pot when the vegetables are, not at the end.