Heat one or two greased square griddle pans on the stove over medium heat.
Place eggs, milk, vanilla, and cinnamon in a square baking dish and whisk very well.
Place each slice of sourdough bread in the egg mixture. Use tongs to flip the slices over to make sure all sides of each slice of bread are coated in the egg mixture.
Place each slice of bread coated in the egg mixture on the hot greased griddle pan. Cook for 2-3 minutes.
Use a spatula to flip each slice over and continue cooking over medium heat for another 2-3 minutes, or until each side is lightly browned.
Serve right away or place on a baking sheet in a warm oven to keep warm until ready to serve.
Notes
*Nutrition facts are an estimate and will vary based on the thickness of slices, the kind of milk used, and how much egg mixture is absorbed by the bread.
The slices of sourdough should not sit in the egg batter for more than a few seconds. If you leave the bread to sit in the batter, your French toast will be soggy.
Cook time will vary depending on the thickness of the slices of sourdough used, as well as how much of the egg mixture the bread soaks up.
Use avocado oil spray or butter to grease the skillet or pan(s).Whisk the egg batter very well using a fork or whisk to blend the egg whites and yolk together well. This will help keep the French toast from being too 'eggy.'If you like French toast to be more 'eggy,' lightly whisk the egg batter.
If you don't like French toast to be 'eggy' at all, you can increase the amount of milk.
If you like French toast a little darker and crispy on the sides, cook each side a little longer.
I've heard that sprinkling a small amount of sugar on each side before cooking it will help to caramelize it, but I haven't tried it because I try to minimize the amount of added sugar in main dish recipes like this one.
If you will be serving this French toast to kids and don't have maple syrup, fresh fruit, or whipped cream to top it with, consider adding a tablespoon of maple syrup to the egg batter to give it a little sweetness to balance out the sour taste from the sourdough bread.
To enhance the flavor even more, consider adding a dash of ground nutmeg in addition to the cinnamon.
Buying a whole loaf of sourdough and slicing it yourself using a bread knife will help you get the perfect thickness to make sourdough. While I don't like to use very thick slices of bread to make French toast, sometimes pre-sliced store-bought bread are sliced too thin.
If you only have one pan or skillet to use, it will take longer to make 10 pieces of French toast.