In a large enamel-coated cast-iron Dutch oven, combine peanuts and water to cover. Let stand for 30 minutes. Drain. Return peanuts to Dutch oven.
Add beer, mustard, salt, and red pepper; bring to a boil over medium-high heat. Reduce heat; simmer, partially covered, until tender, about 6 hours, adding water, 1 cup at a time, as needed. (Peanuts should stay submerged in liquid.) Let cool in cooking liquid. Refrigerate for up to 1 week in an airtight container.
Recipe by Southern Cast Iron at https://southerncastiron.com/beer-mustard-boiled-peanuts/