In a small bowl, mix together the paprika, garlic powder, cumin, chili powder, salt and brown sugar. Rub this all over the pork shoulder.
Brown the pork shoulder on all sides over high heat on the stove. Place pork shoulder in the slow cooker with the onions and turn the heat to low for 8 hours.
After 8 hours is up, shred the pork with a fork and mix it all up with the "pan sauce" that's inside the slow cooker. Keep it warm while you make the chimichurri sauce.
To make the sauce, blend together the cilantro, olive oil, garlic, sugar, salt and serrano in either a high power blender or a food processor.
To assemble the nachos, heat your broiler to high. Spread out the tortilla chips on a large sheet pan. Sprinkle beans over the chips then add the cheese (a few big handfuls should do the trick). Place under the broiler for about 2-3 minutes until the cheese has melted. Top with the shredded pork, some of your favorite salsa, a few big dollops of Greek yogurt and the chimichurri sauce.