Move an oven rack to about 4" away from the broiler. Preheat broiler to high.Brush cod fillets with half the oil, and then sprinkle with salt and pepper. Add remaining oil to an oven-safe pan or casserole dish.Slide the pan into the oven.
Cook fish for 8 minutes per 1" thickness, or until the fish is opaque and the edges are beginning to flake apart. (Thinner fillets cook faster than 8 minutes, so make sure to watch the cod closely.)Do not flip the fish halfway through cooking.
When the fish is done cooking, flake it apart with a fork. If the fish has a skin, discard the skin.Set fish aside and wipe out the pan. Turn off the broiler.
Preheat oven to 400°F.Add chips to a large oven-safe pan or sheet pan.Top with the cheese.
Add the fish, avocado, jalapeños, tomatoes and green onion.
Slide the pan into the oven and cook nachos for about 8 minutes, or until the cheese melts.While the nachos cook, stir together all the garlic yogurt sauce ingredients.When the cheese is melted, remove nachos from the oven. Add the garlic sauce, salsa and hot sauce (if using), and serve immediately.