Preheat oven to high broil. Peel off outer skin of tomatillos, wash, and place them on a baking sheet along with the jalapeno. Broil for 5-10 minutes until the tops are starting to get charred. Remove from the oven and let them cool down a bit.
Add roasted tomatillos and jalapeno to a blender along with the garlic and blend into a smooth sauce.
Add chicken thighs to a soup pot with tomatillo sauce. Cover and turn heat up to medium. Cook for 15-20 minutes until chicken is tender.
Remove from heat and using two forks, or a fork and knife, break chicken thighs up into bite-sized chunks.
Return pot to medium heat and add cabbage, onion, stock, and seasonings and stir to combine evenly. Cook for 30-40 minutes until cabbage is tender.
(Optional) Serve topped with plain Greek yogurt and diced avocado. Cilantro and jalapeno and also great toppings for this soup!