Mix all the ingredients for the marinade together and pour into a sealable bag with the beef cubes. Massage the marinade into the meat and leave to marinate for a minimum of 30 minutes.
Meanwhile, bring a little water to the boil and add the onion. Simmer for five minutes to precook the onion.
Thread the beef cubes onto eight soaked wooden kebab sticks, alternating with the peppers and the onion. Grill over medium-hot coals or under a grill for 7 to 10 minutes, turning frequently. Serve with potatoes and salad.
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