Susan's Lunch is a favor in Sleeping Dogs. Favors are optional except for this one. They allow Wei to "gain face", giving him experience and unlocking the ability to purchase more clothing, vehicles etc.
The waypoint is locked onto the Golden Koi restaurant. Make your way through the market. As you reach the front of the restaurant, a mass of thugs will be loitering out front, looking to start a fight. Deal with them before you head inside and pay the cashier behind the counter.
Rewards: HK $200, Field Report: Winston Chu