Preheat the oven to 200⁰C – Gas 6. Lightly grease 8 holes of a 12-hole muffin pan. Combine flour, soda and cayenne in a medium bowl. Break the eggs in a small bowl, whisk lightly and then whisk in the milk, the butter and the onion until combined. Pour the egg mixture onto the flour mixture and whisk until the batter is smooth.
Divide half the batter among the prepared holes, top with the combined cheeses, and then the remaining batter. Bake uncovered in a hot oven for about 20 minutes or until the muffins are well risen