Preheat oven to 200⁰C fan forced. Brush flank with 1 tablespoon oil and sprinkle with pepper. Bring to room temperature before cooking.
Place potatoes in a bowl and toss with remaining oil, sprinkle with salt and pepper and place on a baking tray with garlic. Roast for 30-35 minutes until very golden and crispy. Remove garlic after 20 minutes or until just soft.
Place a medium frying pan over high heat and when hot place flank in pan, pressing lightly so meat browns evenly. Cook for 4-5 minutes each side for rare. For medium or medium rare, place in oven for 5-10 minutes to cook to desired doneness. Rest for 10-15 minutes and slice thickly.
Make aji sauce by finely chopping coriander roots and stems and half the leaves. Reserve remaining leaves to garnish. Combine with remaining ingredients in a small bowl. Season to taste.
Serve sliced flank with aji sauce spooned over, roast potatoes, roast garlic and reserved coriander leaves, if desired.
Make the sauce in a blender or small food processor for a smooth green sauce.
Try aji sauce with red chillies, or lemon instead of lime