Chop the avocado, green apple, and red apple into thin slices.
Marinate the sirloin steak with salt, ground black pepper, oregano, and thyme. No need to let it rest.
Sear garlic with butter on a cast-iron pan over high heat.
Wait until the pan is sizzling hot and add the sirloin steak in. Let it cook on the first side for 2 minutes.
Turn the steak and let it cook for 1 more minute.
Remove the beef from heat.
Make the dressing: Whisk together olive oil, lemon juice, mustard, and apple cider vinegar. Set aside.
Slice the sirloin into thin slices.
Assemble the beef on a dish together with the remaining ingredients: avocado, green apple, arugula, feta cheese, red onion, and romaine lettuce.
Drizzle the dressing all over the salad. Toss to coat.
Serve and enjoy.