Heat Oil: Heat the oil in a heavy bottomed 5-quart Dutch oven over medium heat.
Cook Onion: Once hot, add the onion with a pinch of salt and pepper and cook for 3-4 minutes until softened but not browned, stirring occasionally.
Add Garlic and Seasoning: Add in the garlic and cook for an additional 30 seconds, then add the cajun seasoning and cook for another 30-60 seconds, stirring constantly.
Simmer Soup: Add everything else to the pot (except the cornstarch, if using), season with another pinch of salt and pepper, and bring it to a light simmer. Cover and cook for 15-20 minutes until the potatoes are fork tender.
Blend Soup: Transfer about 3 cups of the soup to a blender and blend until smooth. Be careful to allow steam to release while you blend. Pour the soup back into the pot with the unblended portion and stir to combine.
Add Cornstarch (Optional): For additional thickening, mix 2 tsp cornstarch with 2 tbsp water until combined then whisk it into the soup.
Season and Serve: Taste and adjust for seasoning if needed and serve immediately.