Gather all ingredients.
Grease a 2-quart rectangular baking dish.
Whisk eggs, milk, maple syrup, brown sugar, vanilla, cinnamon, and salt together in the prepared dish.
Stir in oats and amaranth. If desired, stir in flavor variation mix-ins (see below).
Cover and chill at least overnight or up to 3 days.
Preheat the oven to 350 degrees F (180 degrees C). Uncover and stir to re-incorporate all of the ingredients.
Bake, uncovered, until golden brown and the center is set, 45 to 55 minutes.
Let stand 10 minutes before serving. Serve warm.
