To make this rustic, hearty quick bread, we lean on a combination of whole-wheat and all-purpose flours , but also include muesli; it’s a simple way to include a mix of nuts and other grains. We first soak the muesli in buttermilk to hydrate and soften the ingredients so they disappear into the crumb of the loaf. Any variety of muesli—even those made with dried fruits and seeds—works, but use one that is not sweetened with added sugar. Bob’s Red Mill, Familia and Alpen are good, widely available options. But note that each has a different weight per cup, so it’s best to weigh the muesli. If weighing is not an option, use 1 cup of Bob’s Red Mill, 1¼ cups Familia or 1½ cups Alpen. Baking the loaf in a cast-iron skillet yields a beautifully browned bottom crust, but a parchment-lined baking sheet gets the job done, too, albeit with a slightly lighter crust. The soda bread is best the day it’s baked, but leftovers will keep in an airtight container for up to three days.