In a large pot, cook the onions in oil over medium-low heat for 10 to 15 minutes, until translucent. Add the garlic and cook for 1 more minute. Add the bell peppers, chili powder, cumin, red pepper flakes, cayenne and salt. Cook for 1 minute. Crush the tomatoes by hand or in batches in a food processor fitted with a steel blade (pulse 6 to 8 times.) Add to the pot with the basil. Bring to a boil, then reduce the heat and simmer, uncovered, for 30 minutes, stirring occasionally.
Add the cooked chicken to the chili and simmer, uncovered, for another 20 minutes.
Serve with the toppings, or refrigerate and reheat gently before serving.
Notes
If preparing this recipe as DAIRY FREE, don't serve with sour cream or cheese. If preparing this recipe as GLUTEN FREE, choose tortilla chips that are known to be GF.