Preheat the oven to 200°C/400F fan.
Put the bacon on a grill pan (the type with a rack to let the fat drain) and put it in the oven. Set a timer for 25 minutes.
Brush a muffin tin with the oil, coating each muffin space well.
Beat the eggs in a large bowl and add the spinach, tomatoes and salt and pepper. Spoon into the muffin tin.
Bake for 20 minutes or until just cooked through.
While they are in the oven, split the muffins (I use a fork to do this so you get nice crunchy bits as they toast) and toast either using your toaster or on another tray in the oven if you have space. (This step is optional - see notes).
When the eggs and bacon are both cooked remove from the oven and allow to cool a little (the eggs will sink, don't worry!)
Make up your muffins by filling them with a slice of cheese, the baked egg and 2 bacon rashers.
Wrap up tightly, label and store in the fridge or freezer.