If you are making homemade dressing on the day of, do this first, then place in the fridge so the flavors have a few minutes to meld together. Preheat a large (12-inch diameter or larger) cast iron skillet over medium heat.
(Optional step. Skip to step 3 if not cooking onions). Add 1 tablespoon of olive oil to a hot skillet over medium heat, followed by the red onions with a big pinch of salt. Cook stirring occasionally to prevent burning for about 5-7 minutes, until the onions are very soft with some golden brown edges. Remove from the skillet and set aside.
Add the remaining 1 tablespoon of olive oil to the skillet, and increase the heat to medium-high. Season the steaks generously with salt and pepper on all sides. Add to the hot skillet in a single layer, and leave it alone so it can develop a nice crust.
Once the steak naturally releases itself from the pan, flip to the other side and cook. This should take about 2-4 minutes depending on the thickness of the steak. Our steaks are quite different in shape, so each of them had unique cook times. Once the steak is flipped over, begin monitoring the temperature with a meat thermometer. Remove the steak from the heat once it has reached a few degrees below your desired doneness. We like our steak between a rare and medium-rare, so we take it off the heat somewhere between 118°F -123°F.
Make sure to let the steak rest for 5-10 minutes before slicing against the grain! This helps lock in all the juice, so your steak stays nice and tender. Set aside while you build the salad.
Layer on mixed greens and arugula, then top with tomatoes, grilled onions, avocado slices, cucumbers, feta cheese, and strips of steak. Drizzle with dressing of choice. Enjoy!
This recipe is best enjoyed right away, and we don't recommend making it in advance. However, if you need to save it, store the different ingredients in separate airtight containers in the fridge for up to 3 days. We don't recommend refrigerating the tomatoes, however, so try to add those fresh to the salad. Do not freeze.