In a large stock pot, melt butter and diced onion. Allow to cook on medium high heat for 2-3 minutes until soft and fragrant.
Add in chicken stock. Allow to simmer for 5 minutes.
While soup stock simmers, dice the leftover corned beef, carrots, potatoes and cabbage. Leave a few large pieces of potatoes.
Add corned beef and vegetables to the soup stock. Season with salt, pepper and garlic powder.
Allow to simmer on medium high heat for 10 minutes.
Remove one cup of broth and the large chunks of potatoes. Using an immersion blender, blend until smooth. Return to stock pot.
Add in half and half. Stir through. Add in cornstarch and allow to simmer for another 10 minutes.
Remove from heat, serve and enjoy!