Preheat electric griddle to 350ºF (if using). In a small bowl, combine cinnamon, nutmeg, and sugar and set aside.
Melt butter on the griddle or in a 12-inch skillet over medium heat. Whisk together the cinnamon mixture, eggs, milk, and vanilla. Pour the mixture into a shallow container such as a pie plate or dipping tray.
Dip each side of the sourdough bread in egg mixture, then place it on the griddle or skillet without crowding. Cook french toast slices until golden brown, then flip to cook the other side.
See tips below to keep them warm until serving. Top with butter and drizzle with warmed maple syrup before serving.
Notes
Keeping Sourdough Bread French Toast WarmSet your oven to 175ºF. Put an oven-safe plate, casserole, or baking sheet in the oven to preheat. Add your French toast slices to the oven as soon as they're finished cooking, and keep them in there until you're ready to serve. Cover the plate with a slightly damp, clean kitchen towel to keep the sourdough French toast from drying out.