Preheat the oven to 400 degrees . Using a food processor or a stand mixer fitted with the meat grinder attachment, grind the meat. Preheat a cast-iron skillet over medium-high heat. In a bowl, mix together the meat, onion, parsley, egg yolk, salt and pepper. Form the mixture into 4 patties.
In a 12- to 13-inch casserole, arrange the tortilla chips in an even layer. In a small pan or in the microwave, heat the refried beans and stir in the beef stock to thin them a little; season with the cumin. Dollop the refried beans on top of the chips, then top with the tomatoes and cheese. Bake in the center of the oven until the cheese has melted, 12 to 15 minutes.
In a large, hot skillet, melt the butter. Add the beef patties and cook, turning once for 2 to 3 minutes. Flip and cook for 2 more minutes, then remove the patties and arranged the meat on top of the nachos. To serve, dot the nachos with the sour cream mixture and a sprinkling of extra chives.