Preheat your oven to 200° F. This is how you'll keep the french toast warm while cooking the rest in batches.
Beat together eggs, flour, milk, cinnamon, nutmeg, vanilla, and brown sugar until well combined.
In a very large pan ( or two 9x12 pans ), pour half of the egg mixture. Place the bread slices in a single layer in the pan, and then poor the the rest of the egg mixture evenly over them. Let sit for 30 minutes.
In a large 12" skillet, melt ½ tablespoon of butter over medium low to medium heat. Place two slices of the french toast in the pan on medium low heat, cover, and cook for 3 minutes. Then flip both slices and cook for 3 more minutes uncovered. Place the pieces of toast on a sheet pan and place the pan in the 200°F oven to keep them warm.
Repeat, cooking french toast in the skillet and adding cooked pieces to the warmed oven until all slices are cooked.