Add onion to skillet and cook 1 minute, stirring frequently. Stir in lemongrass, garlic and chile. Cook 30 seconds, stirring constantly. Add broth, 2 tablespoons fish sauce and the oyster sauce. Heat to boiling, stirring occasionally. Reduce heat to medium and simmer 1 minute longer. Return beef to skillet and cook 30 seconds, stirring constantly. Serve with rice.