1. Preheat a non-stick griddle pan. Alternatively, use the barbeque. Wipe the beef dry and cut into 1-inch thick steaks. Cook over medium-high heat for no more than 3-4 minutes on each side, or until the meat is cooked to your liking. Slice thinly.
2. Prepare the dressing by coarsely pound garlic and chillies using pestle and mortar or a food processor. Scoop the mixture into a dish and add fish sauce, lime juice, and sugar. Stir until the sugar dissolves (or add a small amount of warm water to speed up the process).
3. In a mixing bowl, combine the beef, onion, cucumber, bean sprouts, baby watercress, spring onion. Pour over the dressing and add the salad . Toss to combine.
4. Transfer to a serving bowl and garnish with fresh coriander leaves, fresh mint, and fresh Thai basil (or any other leaves you prefer) and serve immediately.