Cook the broccoli and cauliflower in boiling water until almost mushy. Or steam it. Steaming is probably better.
Meanwhile, slowly cook the chopped onion in a skillet with olive oil (or butter).
Drain the veg well, then add to the skillet. Cook for about 10 minutes, longer if you can manage it.
Chop up the corned beef into chunks and toss that in.
Season with salt and pepper and see how long you can wait until you just want to eat. Or wait until it's crispy.