Preheat grill. Spread butter on both sides of steak. Sprinkle salt and pepper on both sides of steak.
Grill 3-4 minutes per side, or to desired doneness. Remove from grill and let stand 5 minutes.
While steak is standing, arrange spinach evenly among 5 salad plates, bowls, or prep containers. Top evenly with halved tomatoes, crumbled feta, and peeled and halved hard boiled eggs. Cube steak and add to salad.
Add all ingredients to a shaker cup or blender. Shake or blend to combine. Arrange vinaigrette evenly over salad, or, if prepping ahead, into water-tight containers.