Matt if you're reading, you might want to avert your eyes... the onions might offend.
This was a surprisingly nice and easy lunch just now:
In a frying pan or wok, heat a little oil and fry the diced onion on a medium heat for 3 or 4 minutes until starting to soften. Break the egg into the middle of the pan. Break the yolk, but you don't need to stir it all in. Using a spatula, draw the egg/onion into a neat-ish circle towards the middle of your pan (about the same size as your bread!). Sprinkle a little bit of black pepper on top, and turn down the heat a tiny bit. Let this cook for 3 or 4 minutes until the egg is set all the way through. (The onion will caramelise a bit.)
Butter your bread slices and put a layer of spinach leaves on one. When the egg/onion is ready slide it out of the pan, on top of the spinach. Let it sit for a few seconds (to let the steam escape) before putting the other slice on top. Nice sandwich.