Pour broth into big, heavy saucepan and place over medium-high heat. Stir in Sriracha.
While broth heats, dice chicken into bite-size pieces; ditto the avocado. Slice scallions, separating white and crisp green parts.
When broth is simmering, stir in chicken and white part of scallions. (Don’t just drop the diced chicken in without stirring! It’ll congeal into a lump in the bottom of the pot. Stir it in.) Crush in garlic. Bring back to a simmer, turn burner to low, and let simmer for 10 minutes.
Salt and pepper the soup to taste. Ladle into bowls. Now divide avocado and sliced green scallion shoots among bowls, and serve.
NUTRITION: 277 calories; 13 g fat; 34 g protein; 6 g carbohydrate; 2 g dietary fiber; 4 g net carbs per serving.