Combine beef, 1 tablespoon Indonesian sweet soy sauce and half the garlic, cover and set aside.
To make the satay sauce, very finely chop the onion in a food processor. Heat 1 tablespoon oil in a frypan over medium heat. Cook onion stirring for 4-5 minutes until softened. Stir in chilli, lemongrass and remaining garlic, then cook for 2-3 minutes until fragrant. Crush peanuts in a food processor and then add to the pan with sugar, lime juice, coconut milk and 1/2 cup water. Simmer for 8-10 minutes until thickened and reduced, stir in remaining 1 tablespoon Indonesian sweet soy sauce.
Heat 1 tablespoon oil in a separate pan over medium-high heat. Cook half the beef, turning for 5 minutes. Add 1 tablespoon of the satay sauce and cook tossing to coat the beef for a further 2 minutes or until caramelised. Repeat with remaining beef, oil and satay sauce.
Divide beef, cucumber, coriander, coconut and extra chilli and onion among flat breads. Serve with lime wedges and remaining satay sauce.