Heat oil in heavy large pot over medium-high heat. Add onions and garlic; sauté until tender and golden, about 9 minutes. Add squash; stir 2 minutes. Stir in chili powder and cumin. Stir in beans, broth, and tomatoes with juices; bring to boil. Reduce heat and simmer, uncovered, until squash is tender, about 20 minutes. Stir in spinach; simmer until wilted, about 4 minutes longer. Season to taste with salt and pepper. Ladle chili into bowls and serve.