Thinly slice the beef across the grain. Mix lemongrass, garlic and 2 teaspoon of the peanut oil together in a bowl. Add the beef and stir to coat.
Put the peanuts in the cold wok over high heat. Stir-fry for 3-5 minutes until peanuts are warmed through. Remove to a plate. Wipe wok clean.
Heat the wok over high heat until hot. Add 2 tsp oil and swirl to coat the wok. Add one third of the beef and stir-fry for 30 seconds until sealed. Remove to a clean bowl. Repeat with oil and beef in two more batches.
Add remaining 2 tsp oil then the snow peas and capsicum. Stir-fry 1 minute. Return all the beef and any juices to the wok. Stir-fry 30 seconds. Scatter over the peanuts. Serve over rice.