Heat a large pot over medium heat and add a splash of oil. Once hot, add the diced onion and sauté until it becomes translucent and fragrant, about 3-4 minutes.
Add the minced garlic to the pot and cook for another 30 seconds until fragrant, stirring constantly to prevent burning.
Stir in the cubed chicken breasts and cook until they develop a slight golden color on the outside, about 5 minutes.
Pour in the chicken broth and bring the mixture to a gentle simmer, skimming off any foam that rises to the surface.
Add the cooked corn, cumin, chili powder, oregano, and black pepper, stirring well to combine all the flavors.
Reduce the heat to low and let the chili simmer uncovered for about 20 minutes, allowing the flavors to meld and the chicken to cook through completely.
Use an immersion blender or transfer the mixture carefully in portions to a blender, then blend until smooth and velvety, creating a creamy base.
Return the blended soup to the pot if using a blender, then stir in the lime juice to brighten the flavors and adjust seasoning if needed.
Spoon the hot white chicken chili into bowls and dollop with sour cream; garnish with sliced green onions for a fresh crunch.