McDonald's is investing $100 million to bring customers back to stores after an outbreak of E. coli food poisoning tied to onions on the fast-food giant's Quarter Pounder hamburgers. The investments include $65 million that will go directly to the hardest-hit franchises, the company said. The U.S. Centers for Disease Control and Prevention has said that slivered onions on the Quarter Pounders were the likely source of the E. coli.