Dry the steak well and season to taste with salt and pepper. Heat a large cast iron pan over high heat. Sear the steak, about 5 minutes per side, or to desired doneness. Set aside and let rest for at least 5 minutes. Do not clean the pan.
Add the mushrooms to the now empty pan and cook until they are well browned, 8-10 minutes. Do not crowd the pan or the mushrooms will not brown properly. Add garlic and a pinch of thyme in the last 5 minutes of cooking. Set aside
Make the dressing by pureeing all dressing ingredients until completely smooth.
To assemble the salad, slice the steak against the grain Place atop the lettuce and top with tomatoes, olives, and pumpkin seeds. Drizzle with the dressing and serve.