Preheat oven to 450 degrees F. On each of 2 large, ovenproof platters, arrange 1/4 of the chips. Top each chip platter with 1/4 each of beans, green onions, and cheese. Repeat (you should have 2 layers on each platter). Bake nachos until cheese melts (about 8 minutes), or microwave on high 4 to 6 minutes.
Meanwhile, in a medium bowl, mix tomatoes, red onion, cilantro, garlic, lime juice, and salt.
Top nachos with tomato mixture. Mound sour cream in the middle, and garnish with a cilantro sprig.