Made this tonight using fillets. I'm allergic to bleu cheese and didn't have any feta, so I subbed in some crumbled goat cheese. I used a heavy bottom stainless steel skillet for cooking both onions and the steak. The onions were amazing and had just the right amount of sweetness to them. My husband doesn't normally like a lot of onion but he ate all that was on his plate. I did cheat a little with my steak. I used kosher salt and sprinkled it liberally on both sides of the steak. I got this tip from another blog about pan searing steaks. I got a beautiful crust on both sides and didn't have to add anything else to the steak. In fact, even without the onions, the steak tasted amazing! I'm so happy to have tried this recipe; I now have a go to recipe to make steak for one when my husband is deployed!