Preheat the oven to 180°C/350°F/Gas mark 4 or 200°C/400°F/Gas mark 6.
Grease a large muffin tin (or regular muffin tin).
Saute mushrooms and spring onion in olive oil for a few minutes to soften the mushrooms.
Add spinach and wilt.
Set aside to cool.
Whisk the eggs with the pesto.
Portion the veggies evenly between muffin wells. If using a large muffin tray, it will make 1 per serve (4 in total), if using a regular muffin tray you may get 2-3 per serve or 8-12 in total.
Pour in the eggs and top with crumbled feta.
Bake for 15-20 minutes or until golden on top and springy to the touch.
