1. Warm your oven to 200 degrees.
2. Heat some oil in a large skillet over medium heat. Add the bell pepper, onion, and some salt. Cook, stirring constantly, until just tender; about 3 minutes.
3. Create a single layer of half the chips on a sheet pan. Top with about half the cooked vegetables and a third of the cheese.
4. Spread the remaining chips on top of the first layer. Sprinkle with the rest of the cooked vegetables, cheese, and jalapeños (if using). Bake until the cheese is melted and just starting to turn golden brown, for 8 to 10 minutes.
5. Top with scallions, cilantro, and avocado. Serve with salsa and sour cream, if desired.