Heat the oil in a small frying pan. Peel and slice the onion, and add to the pan, followed by the garlic. Cook for 2 minutes. Drain and rinse the black beans and put them in. Season well with salt and black pepper, and cook for a further 3-4 minutes on a medium heat, stirring occasionally.
Meanwhile, halve the cherry tomatoes, and slice the pepper into thin strips. Remove the stone from the avocado and cut into 1cm chunks. Combine the tomato, pepper and avocado in a small bowl, and season with salt and black pepper. Squeeze over the lime juice and toss everything to combine.
Heat a second frying pan, and gently toast the tortillas on each side until just turning brown.
Roughly chop the coriander leaves and mix them through the tomato and avocado mixture.
Layer up the toasted tortillas with beans and onions, then the tomato, pepper and avocado mixture, and serve immediately.