Preheat oven to 170c/325f/gas 3
Melt butter in saucepan and fry onion. Set aside.
Put ingredients in a large bowl
Beat egg and mix with milk in a separate bowl
Mix/cheese mix an electric whisk and slowly add egg/milk mixture. Then add spinach and mix all together, stopping to take mixture out of the whisk at times
Spoon into cupcake paper cases in a muffin tray. Fill 3/4 full. Bake for 30-35 mins.
Remove from oven, allow to cool for 5 mins in muffin tray and then place on rack to cool fully for a few hours. Best eaten first the following day.