Add the oil to a large frying pan and fry the bacon until browned, 2-3 minutes on each side. Transfer to a plate to cool.
Using about 1 tablespoon of the bacon fat from the pan, fry the onion and red pepper until softened, about 5 minutes. Then add the spinach and stir until wilted down.
Preheat the oven to 350 F and grease a rectangular oven dish with a little butter or cooking spray.
Chop up the bacon into small pieces.
Add the eggs to a large mixing bowl with the salt and pepper and beat well with a whisk.
Add the cooked onion, pepper, bacon, and cheese to the eggs, and stir to combine.
Transfer the mixture to your prepared oven dish and bake for 40-50 minutes, or until the egg is set and starting to go golden around the edges.