Cook the onions in the 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, zucchini, 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.