Rinse 3 cups dried pinto beans under cold water and remove any debris. Place them in a 6- to 8-quart slow cooker.
Dice 1 medium yellow onion and finely mince 1 medium jalapeno and prepare 4 teaspoons minced garlic. Add them to the slow cooker alongside the beans.
Pour in 9 cups water and add 1 teaspoon salt, stirring to combine. Cover with the lid and cook on low for 8 to 10 hours, or until the beans are very soft.
Drain the beans in a colander, reserving some of the cooking liquid. Transfer the beans to a food processor or return them to the slow cooker. Use a food processor or a potato masher to puree the beans to your desired consistency, adding reserved cooking liquid as needed for a smoother texture.
Using a food processor or a potato masher, puree the beans.
Stir in 1/2 cup salted butter until completely melted. Serve hot and enjoy!