1.Combine 2 tablespoons oil, half the chilli and one-third of the garlic in a bowl and brush over the beef. Season, then set aside at room temperature for 30 minutes.
2.Meanwhile, heat remaining 1 tablespoon oil in a saucepan over medium heat. Add eschalot and cook, stirring, for 3 minutes or until softened but not browned. Add ginger and remaining garlic, and cook for a further 1 minute or until fragrant. Add soy sauce, coconut milk and sugar, stirring until sugar dissolves, then bring to a simmer and cook for 1 minute or until slightly reduced and thickened. Add peanut butter and stir until melted and combined. Set aside.
3.Preheat a barbecue or chargrill pan to high. Cook beef, turning, for 10-12 minutes until lightly charred but rare in the centre, or until cooked to your liking. Rest, loosely covered with foil, for 5 minutes.
4.Meanwhile, gently reheat peanut sauce over low heat. Thinly slice beef and arrange on a serving plate with watercress and cucumber wedges. Drizzle over spoonfuls of peanut sauce, and serve scattered with coriander leaves.