Mix the onion, tomato, asparagus and olive oil on an aluminium lined baking tray. Sprinkle with salt and pepper. Roast in preheated oven until veggies are soft, about 10 minutes. Microwave the beans until hot.
Warm taco shells in the oven for 5-10 minutes then stuff shells with veggies, avocado and black beans.
Black beans are not that common but can be found in tins in the organic department in your supermarket, at a health food shop or dry at places like Bin Inn. (Dry beans must be soaked overnight in water.)