Rinse the chilies, remove seeds and stems, and finely chop. Peel and chop the onions. Peel the cucumber, leaving some strips of skin for color, and slice thinly. Rinse the lettuce hearts and spin dry. Set aside.
Make the dressing: Crush the garlic through a press into a large bowl. Add the lemon zest, lime juice, caster sugar, fish sauce, and soy sauce. Stir until the sugar dissolves completely.
Heat a nonstick pan over high heat and grill the beef slices for 1.5 minutes per side for rare. Let cool slightly, then slice into thin strips (3cm long, 5mm x 3mm thick).
Toss the beef strips in the dressing until well coated.
Divide the lettuce and cucumber among 4 plates. Top with the dressed beef and sauce. Garnish with chopped onions and chilies. Serve immediately.
Tip from the kitchen: For a seafood twist, substitute with 200g large peeled prawns, steamed or blanched briefly and served warm.
