If you prefer a raw muesli, mix the grain flakes, oats, and bran, if using, in a large bowl. If you prefer a toasted muesli, mix the grain flakes, oats, and bran, if using, on a rimmed baking sheet and spread the grains so everything is 1/4 to 1/2 inch deep. (You may need 2 baking sheets.) Bake for 6 to 8 minutes, or until the grains look lightly toasted (but not browned) when you stir them with a spoon. Transfer the toasted flakes to a plate or another baking sheet and let cool to room temperature. Dump the flakes in a large bowl.