In a 13-inch cast iron pan, or 9×13 inch oven-safe pan, heat the oil over medium heat, add the diced onions and garlic and saute 5 to 7 minutes, until onion is translucent. Add the ground beef, salt, and pepper, and cook 7-8 minutes until ground beef is browned and fully cooked. Remove from pan.
Add the 3 tablespoons oil to the pan, then add hash browns. Cook 5 to 6 minutes, then flip, and cook an additional 3-4 minutes, or until the potatoes are crispy and cooked through. Place the ground beef and onion mixture on top of the hash browns, then pour on the nacho cheese.
Put the pan in the oven and bake for 15 minutes. While casserole is baking, prepare the toppings.
Once casserole has finished baking, garnish with the lettuce, bacon, tomato, avocado, and jalapeno and serve!