Rinse the dried pinto beans under cold water, then place them in a large bowl and cover with several inches of water.
Soak the beans overnight or for at least 8 hours, then drain in a colander.
Place the drained beans into a large pot or slow cooker and add fresh water to cover them by about 2 inches.
Add the quartered white onion, whole jalapeño, and the garlic clove to the pot; season with a pinch of salt and a pinch of black pepper.
Bring the pot to a rapid boil over high heat for about 30 minutes, then reduce the heat to medium and simmer for about 15 more minutes, or until the beans are tender.
When the beans are cooked, remove from heat and discard the jalapeño and whole garlic if desired, then serve or cool before storing.