Preheat oven to 400°. In a large frying pan, heat oil over high heat. Cook onion, stirring often, until softened, about 4 minutes. Add bison and cook, stirring well to break up meat, until no longer pink, about 4 minutes. Season with pepper.
Meanwhile, lay half the tortilla chips evenly in the bottom of a shallow baking dish. Spread half of bison mixture over chips. Pour 1/3 cup salsa over the meat, followed by half the cheese. Repeat layers with 1/3 cup more salsa and remaining meat and cheese. Bake until cheese melts and chips start to brown, about 6 minutes.
Serve nachos with lettuce, avocado, tomatoes, remaining 1/3 cup salsa, and the sour cream.