Heat the oven to 180C. Heat the taco shells upside down on a tray for 1 to 2 minutes. Heat the olive oil in a non-stick frying pan and add the beans, stirring well with a wooden spoon. Add the salsa, cumin, sea salt, pepper, Vegemite and water, stirring well.
Mash the beans roughly in the pan with a potato masher.
Pile some shredded lettuce into each taco shell, spoon the beans on top, and add a dollop of sour cream, and an extra dollop of salsa. Scatter with diced avocado and jalapeno chillies and serve.