Sort through peanuts and remove any debris by rinsing with cold water.
Place peanuts to slow cooker. Add kosher salt, Cajun seasoning, crushed red pepper flakes and jalapeño. Add about 6 cups of water or enough to cover the peanuts as they float. Cover and cook on low for 22-24 hours.
Drain and serve warm or cold.
While some people recommend soaking the peanuts for 6-8 hours in salted water, there is no need to do this, unless you are using raw peanuts.
The peanuts will float at first, but will eventually settle as the slow cooker starts to cook them.
You control the amount of heat by the amount of crushed red pepper flakes and jalapeño you add.
Some like boiled peanuts harder while others like them softer. You control this by the amount of time you cook them, with the longer you cook the peanuts the softer they become.