In batches, cook the eggplant in oil until browned on all sides in a Dutch oven. Remove with a slotted spoon, keep warm. To the same Dutch oven, add onion, cook until tender. Add garlic, cook one minute longer. Add crushed tomatoes, stirring to loosen brown bits from pan, basil, sugar (if using), oregano, salt and red pepper flakes, if desired. Bring to a boil. Reduce heat; simmer, uncovered, until thickened, 10-15 minutes, stirring occasionally.