Heat the oil in a skillet on the pan and add the onion and garlic. Fry lightly for 3-4 minutes until the onions start to soften. Add the minced beef and cook on a higher heat until the meat has browned.
Spoon the onions, garlic and minced beef into the slow cooker. Add the quinoa, stock, tomatoes and spices and stir well. Cook on low for approximately 3 hours until the quinoa is cooked and has absorbed all the liquid.
Stir in the black beans and corn and sprinkle the grated cheese on top. Put the lid back on the slow cooker and leave it for another 10 minutes until the cheese has melted before switching off the slow cooker.
Fill each lettuce leaf with two large spoonfuls of the chili quinoa and top with a small handful of chopped tomatoes, avocado and spring onions.
Serve immediately with salsa and sour cream (optional).