In a large pot, heat the sausage over medium heat, breaking it up as it browns.
Add the diced onion and minced garlic to the pot; cook until the onion is translucent.
Pour in the chicken broth and scrape the bottom of the pot to release any tasty browned bits.
Stir in the diced tomatoes, including their juices, and let the mixture simmer.
Add the kale and season the soup with red pepper flakes, salt, and pepper.
Let the soup simmer for another 5-10 minutes, stirring occasionally, until the kale is tender.
Taste and adjust seasoning if necessary before serving hot.