Heat a griddle pan. Cut the cornbread into cubes and place on the griddle pan. Turn every 30 seconds or so until each side has been grilled and is nicely golden. Remove and set aside.
Thinly slice the red onion into half moons. Place into a small bowl. Add a pinch of salt, the juice of half a lime and 1 tablespoon of red wine vinegar. Leave to steep for 10 minutes or so (though longer won't hurt).
Place the black beans in a large bowl. Halve or quarter the tomatoes, depending on size and dice the avocado. Add these to the bowl.
Using a stick blender, blitz the coriander with the oil. Finely chop the jalapeño and add this to the coriander oil along with the cumin and a pinch of salt.
Add the cornbread cubes to the bowl with the black beans, tomato and avocado. Pour in the steeped red onion along with all of the juices. Pour in the coriander oil. Mix everything together well. As with traditional panzanella, this is a pretty hardy salad and is perfectly fine (arguably better) if it's made in advance. Season to taste.