1. In a medium bowl, whisk the soy sauce with the vermouth, cornstarch and sugar. Add the sliced sirloin and toss well to coat.
2. Heat 1 tablespoon of the oil in a large skillet until almost smoking. Add half of the garlic, ginger and jalapeño and stir-fry over high heat until fragrant, about 30 seconds. Add half of the meat and stir-fry until browned in spots, about 2 minutes. Transfer the meat to a plate. Return the skillet to high heat and repeat with the remaining oil, garlic, ginger, jalapeño and beef.
3. Add the bok choy to the skillet, season with salt and pepper and cook over high heat, stirring, until crisp-tender, about 4 minutes. Return the meat and any accumulated juices to the skillet. Add the stock and bring to a boil. Stir in the sesame oil and serve.
Serve With: Steamed jasmine rice.