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First, make the chimichurri base. Put the shallot in a mini food processor with the garlic, herbs and oil. Pulse until finely chopped but still with some texture – like a chunky pesto. Put 1 1/2 tbsp in a small bowl and set aside. Mix the red wine vinegar, chilli and 1 tbsp water into the remaining mixture and season. Chill until assembling.
Stir the cumin seeds and lime juice into the reserved 1 1/2 tbsp chimichurri base, then spoon all over the steak strips in a non-metallic dish. Give everything a good mix to coat, then marinate in the fridge for at least 30 mins, or up to 2 hrs.
Rinse the rice and put in a saucepan with triple the volume of water. Bring to the boil, then lower the heat, cover and simmer gently until cooked. Drain any remaining liquid, then spread on a tray to cool quickly.
When the rice is almost cool, stir in half the chimichurri sauce with the black beans, shallots, avocado and cucumber.
Thread the beef onto 6 large or 12 small skewers and heat the grill or a griddle pan to high. Grill for 3-4 mins each side, or griddle for 1-2 mins each side, depending on how well cooked you like your steak. Serve the rice on a platter with the skewers, lime wedges, yogurt mixed with lime zest and black pepper, and the remaining chimichurri.