Blanche the peas - First, bring a saucepan of water to the boil and add your frozen peas. Simmer for 3 minutes then drain and leave to cool for a few minutes.
Preheat oven and prepare the muffin tin - Preheat your oven to 350°F (175°C). Grease a 12-cup muffin tin with the olive oil or line with silicone muffin cases.
Make the egg mixture - In a large mixing bowl, crack the eggs and add the milk, a small pinch of salt and pepper to taste. Whisk with a fork until just combined. Avoid over whisking.
Add the vegetables and herbs - Add the peas, chopped spring onion, and chopped parsley to the egg mixture and stir until evenly distributed.
Fill the muffin tin - Pour the egg mixture evenly into the prepared muffin tin, filling each cup about 3/4 full. Sprinkle on the crumbled feta cheese, distributing it evenly.
Bake - Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the frittata muffins are set and the tops are lightly golden brown.
Cool - Once done, remove the muffin tin from the oven and let the frittata muffins cool for a few minutes before carefully removing them from the tin.
Serve - Breakfast frittata muffins are best served warm or at room temperature.