Lightly oil a 12-hole muffin tin. Cut off any rind and excess fat, then finely chop the bacon and dry-fry it in a pan with the onion over a moderate heat for 3–4 minutes until the bacon is turning crisp. Cook the sweetcorn in boiling water for 2 minutes to soften.
Put the cornmeal, flour and baking powder in a bowl and mix together. Add the sweetcorn, cheese, bacon and onions, and stir in.
Whisk the milk with the eggs and oil and add to the bowl. Stir gently until combined, then divide among the tin sections.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 15–20 minutes until golden and just firm. Loosen the edges of the muffins with a knife and transfer to a wire rack to cool.