Heat oil in heavy bottomed large pot over medium heat. Add onions and sauté until tender and golden, about 10 minutes. Add garlic and cook another minute, or until garlic is fragrant. Add squash; stir; and cook for 4 minutes, stirring occasionally. Stir in chili powder and cumin; cook for 2 minutes. Stir in sriracha, kale, beans, broth, tomatoes, and salt; increase heat to medium-high and bring to boil. Reduce heat and simmer, uncovered, for 45 minutes. Season with additional salt and pepper as needed. Ladle chili into bowls, top with avocado and sour cream (if using), and serve.