Heat the oil in a pan over a medium heat. Combine the steak, red pepper and chilli in a bowl, then add the cinnamon and allspice and mix well. Tip the steak and pepper mixture into the pan and cook for 5-7 minutes. Remove from the heat and leave to cool a little.
Mix the yogurt, lime, thyme and a pinch of salt in a bowl. To serve, arrange the lettuce on a platter and top with the steak, peppers and mango, with the dressing on the side or drizzled over the salad.