Preheat the oven to 375 ℉. Prepare a 9 inch cast iron pan with baking spray or a large baking sheet with parchment paper.
In a small bowl or measuring cup, stir together milk and vinegar and let sit 5 minutes.
Add egg to milk mixture and whisk well.
In a mixing bowl combine flour, baking soda, salt, and butter. Crumble butter with hands or a fork into the flour mixture until it's in pea sized pieces.
Mince onions and chop cheese into small pieces, about ¼ inch cubes (about .5 cm).
Add the onions, cheese, and garlic to the flour mixture.
Pour the milk mixture into the dry ingredients and mix to form a dough.
Once the dough is formed, knead about 10 times, making sure that all flour pockets are gone. Gently form into a ball, use flour if you need to keep the dough from sticking to your hands.
Place the dough ball in cast iron frying pan (or prepared baking sheet) and cut a cross ¼ inch deep into the top of the loaf using a knife.
Bake for 45-55 min, until internal temperature of loaf is 200℉. Let cool for at least 30 minutes before slicing. Enjoy!