In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and cook for 2-3 minutes until it begins to soften.
Add the minced garlic, bell pepper, zucchini, carrot, celery, and jalapeño pepper(if using). Sauté for about 5 minutes, or until the vegetables start to soften and the onion becomes translucent.
Stir in the chili powder, ground cumin, smoked paprika, cayenne pepper, salt, and black pepper. Cook for an additional 2 minutes, stirring constantly to toast the spices.
Add the crushed tomatoes, drained black beans, kidney beans, pinto beans, and corn kernels (if using). Pour in the vegetable broth and stir well to combine all the ingredients.
Bring the chili to a boil, then reduce the heat to low. Cover and simmer for about 20-25 minutes, allowing the flavors to meld and the vegetables to become tender. If you're using fresh or frozen corn, add it during the last 5 minutes of cooking.
Taste the chili and adjust the seasoning if needed. If you prefer a spicier chili, you can add more cayenne pepper at this stage.
Serve the vegetarian chili hot, garnished with chopped fresh cilantro, diced avocado, shredded vegan cheese, and a dollop of vegan sour cream if desired.