With paper towels, gently pat the steaks to remove excess moisture.
Brush a light coat of oil over the fillets, then season with a generous amount of coarse Kosher salt (about 1-1.5 teaspoons per pound) and freshly ground pepper (about 1/2 teaspoon per pound). Set the fillets aside to come to room temperature.
Heat a cast-iron grill pan or skillet over medium-high heat on the stove. (For an outdoor grill, heat to 450–500°F.)
Grease the pan or grates with olive oil or butter, then add the steak to the pan and grill for 4-5 minutes per side, depending on the thickness. (For a nice, perfect medium-rare steak, you typically want to cook it for 4 minutes per side if 1 inch thick and about 5 minutes per side if 1.5 inches thick. You can use a meat thermometer to guide you.) While the steak cooks, mix the baby spinach, toasted walnuts, strawberries, red onion, and crumbled feta in a large mixing bowl. Drizzle your dressing of choice over it all and toss to coat. (I like using my maple balsamic dressing) Remove the steak from the heat when it's done and set it on a cutting board. Let it rest for 5 minutes before cutting into it so the juices set. Then, slice it against the grain.
Transfer the tossed salad onto individual plates, top with steak, sprinkle some extra pepper over it all, and drizzle with a little extra dressing if you wish.