Preheat the oven to 180C. Grease a muffin tin or line with muffin cases. Heat 1 tbsp olive oil in a pan and cook the onion on low heat for 7 minutes, adding in a grind or two of salt. Remove the onion and set aside. Fry the bacon in the same pan for 5 minutes, then place on plate lined with paper towel to soak up excess oil.
In a bowl, sift the flour, baking powder, bicarb soda. Pour in the eggs, kefir and milk. Stir gently with a wooden spoon. Stir in the courgette, onion, bacon. Add in the parmesan.
Use an ice-cream scoop to fill each muffin hole evenly. Bake in the oven for 20-25 minutes. These are great to store in the freezer – simply defrost individual muffins in the fridge the night before and warm in the oven.