Sausage and Kale Soup

In a large pot (6 to 8 quarts), heat oil over medium. Add onion and cook until soft, stirring, 2 to 3 minutes. Add garlic and red-pepper flakes; cook unti fragrant, 1 minute. Add potatoes and broth; bring to a boil. Reduce heat; simmer until potatoes are tender, 10 to 15 minutes.