Place beans, whole onion, and ham hock in saucepan. Add water to cover. Bring to boil, cover and simmer 1 1/2 hours, or until beans are tender. Remove onion and ham hock. Drain beans. Heat lard until very hot. Add to beans and mash with potato masher. Season to taste with salt. Set aside. To make nachos, cut tortillas in quarters and fry in deep hot lard until crisp. Drain and season lightly with salt. Arrange tortillas in single layer on large ovenproof platter. Sprinkle half each of Jack and cheddar cheeses over tortilla chips. Sprinkle chilies over cheeses. Top with dollops of refried beans and gently spread over mixture. (Reserve any extra beans for another use.) Sprinkle with half of remaining cheeses. Top it with tomatoes and cilantro, then with remaining cheeses. Bake at 375F until cheeses are melted and bubbly, about 15 minutes. Serve at once.