Preheat oven to 180C/350F/Gas 4
Heat the olive oil in a small frying pan and saute the onion and bacon until the onion is soft and the bacon browned (10-15 minutes).
Take off the heat, stir in the spring onion or chives, season with a little salt and pepper. Set aside.
Whisk together the eggs and yogurt in a jug. Add a little salt and pepper (remember that the bacon and cheese may be quite salty) and set aside.
Brush a silicon 6-hole muffin tray with oil and stand it on a baking tray. If using a metal muffin tray, you may wish to line with paper cases before brushing with oil.
Divide the bacon and onion mixture between the muffin cups and sprinkle a little Parmesan over each.
Pour the egg mixture into muffin cups and top with the cheddar cheese.
Bake in the oven until a skewer comes out clean (12-15 minutes). To brown the tops further, turn up the heat and cook for another 2 minutes.
Leave the mini frittatas to cool for 3 minutes before turning out. If not eating immediately, cool and refrigerate or freeze.