As bean bags can have small stones in them, check the beans and remove any stones which may be found in them.
Soak the beans in a bowl of water over night. Pour the water out before cooking the beans. If you don't soak the beans over night the cooking time may have to be extended.
Half the onion and quarter the tomato. In a rice ball mold or a large tea infuser put the onion, the tomato and the garlic cloves.
Add the beans to a pressure pot, add the filled rice ball mold or large tea infuser and fill the pressure pot up with water to the max level.
Half the serrano chili length wise and remove the seeds. Add the serrano chili to the pressure pot and also add the salt.
Close the pressure pot securely and cook the beans for about 1 hour (40min to 1.5h depending on how hard or soft you want the beans) and then let the pressure pot cool down before you open it. Squish one bean to see if it's cooked to your liking.