1 Clean shiitake mushrooms and cut off stems.
2 Peel carrots and cut into very fine strips (julienne).
3 Rinse chile pepper, wipe dry and slice into thin rings. (For less spice, remove seeds and ribs).
5 Rinse scallions, pat dry and cut into 3 cm (approximately 1-inch) lengths. Peel garlic and chop finely.
6 Squeeze juice from lime. Mix together 2 tablespoons juice in a small bowl with fish sauce and soy sauce.
7 Pat beef dry, cut into thin strips and season with salt and pepper.
8 Heat oil in a wok (or a deep, heavy pan) over high heat. In 2 batches, stir-fry beef until browned, then remove from wok.
9 Add mushrooms, carrot and scallions to wok and stir-fry until crisp-tender, about 4 minutes.
10 Add chile, garlic and cashews and stir-fry until fragrant, about 1 minute.
11 Return beef to wok, pour in the lime-soy mixture and bring to a boil.
12 Rinse Thai basil, shake dry, pluck leaves and coarsely chop. Add to wok, toss to combine and serve immediately, over brown rice, if desired.