Preheat the oven to 350 degrees. Prepare a 9x13 casserole dish with nonstick cooking spray.
Season the steak cubes with steak seasoning and salt.
In a large skillet over medium-high heat, pour the oil and heat until hot. Add the steak and brown on all sides. The meat does not need to be cooked through.
Add the onion to the skillet and cook until soft and caramelized. Stir in the garlic.
In a large bowl, stir together the browned steak and onions, potatoes, soup, sour cream, and 1 cup cheese.
Transfer the mixture to the prepared casserole dish. Top with the remaining cheese.
Bake for 1 hour, uncovered, or until the potatoes are tender. If the cheese begins to brown, tent the casserole with aluminum foil.