Heat a large pan over a high heat with a drizzle of vegetable oil. Add the broccoli florets and stir-fry for 2 min. Add the white parts of the pak choi and stir-fry for 1 min further. Add the garlic, the green pak choi tops, peanuts and sesame seeds, and stir-fry for 1 min. Season with sesame oil, salt and pepper. Keep warm.