This recipe is highly adaptable. I used many ingredients that I already had in the fridge, and would have added even more if I had them. For the bread, I used a store-bought sourdough that was a couple days old, but you could also use Homemade Bread.
3 cups of bread, cut into one inch cubes
1/4 red onion
1/2 cup spinach
1/4 cup shredded Parmesan cheese
1/4 cup shredded sharp cheddar cheese
1/4 cup milk
couple shakes of pepper
In a large bowl, whisk eggs and milk together. Add the bread and mix until the bread cubes are covered in egg. Refrigerate overnight to allow the bread to be fully soaked. Add the remaining ingredients into the bowl and mix to combine. Bake in an cake or casserole pan for 35-40 minutes at 350 degrees.