Heat the oil over medium/high heat in a large skillet until hot but not smoking. Add the onion and cook for 3-4 mins, until the onion is golden, stirring frequently.
Whisk together sour cream, lime juice, zest and salt.
Preheat the oven to 400 degrees. In a 10 inch cast iron skillet layer one half of the tortilla chips with the shredded chicken, beans, and cheese. Repeat.
Remove pan from the oven and drizzle with the lime crema, sprinkle with cilantro, scallions, tomatoes, avocado and jalapeños. Serve hot.