Add the beans to a large stockpot and cover with cold water. Bring to a boil, then reduce to a simmer and cook until the beans are slightly soft, 30 minutes to 1 hour. Turn off the heat and add the kosher salt and garlic salt.
Add the butter to a large, heavy-bottomed saute pan over medium heat, then add the jalapeno, garlic and onion. Saute until softened, 3 to 4 minutes.
Drain the beans, reserving the liquid, then add the beans to the saute pan and simmer for 10 minutes, adding the reserved cooking liquid if too dry. Season to taste and serve.