Preheat the oven to 180°C and grease three cups of a muffin tin with the olive oil. Slice the leek into thin rings and cut the pepper into small dice.
Fry the leek and pepper for a couple of minutes in a small pan until soft but still colourful. Let them cool slightly and divide among the muffin cups. Whisk the eggs with salt, pepper and finely grated Parmesan cheese.
Pour the egg mixture over the vegetables and bake the muffins for fifteen to eighteen minutes until just set and lightly risen. Let them stand for two minutes before turning out of the tin and finish with parsley. You can easily make them a day ahead; cold they're just as delicious and they deliver a solid dose of protein for the morning.