Place sliced meat into a shallow bowl, and season with salt, black pepper and crushed chillies. Squeeze the lime juice over the meat, and turn until evenly coated. Cover, and refrigerate for 30 minutes.
In a blender or food processor, combine tomatillos and chillies. Purée for 20 seconds.
Heat 1 tablespoon oil in a large frying pan over medium high heat. Carefully pour in tomatillo mixture. Cook, stirring frequently, for 5 minutes. Stir in beef stock. Reduce heat, and simmer for 20 to 30 minutes, or until mixture coats a spoon. Transfer mixture to a serving dish.
Heat 2 tablespoons oil in a large frying pan over high heat. Stir in 1/3 of the beef, and sauté for 1 minute. Transfer to serving dish. Repeat with remaining beef. Meanwhile, heat tortillas/chapatis on the tawa.
To serve, place two tortillas on top of each other. Add desired amount of meat, spoon over some tomatillo mixture. Top with chopped onions, tomatoes, avocado and coriander. Garnish with a wedge of lemon, to be squeezed over taco before eating.