Heat grill or grill pan to medium-high. In a medium pot, combine farro and 3 cups water. Cover and bring to a boil. Reduce to a simmer and cook 15 minutes. Drain.
Season steak on both sides with 1/4 tsp of the salt and 1/8 tsp of the pepper. Grill over medium-high heat 5 minutes per side. Let rest 5 minutes. Meanwhile, in a large bowl, whisk vinegar, oil, mustard, 1/4 tsp of the salt and 1/8 tsp of the pepper.
Thinly slice steak against the grain. Toss in bowl with cooked farro, spinach, tomatoes, shallots, blue cheese and remaining 1/4 tsp salt and 1/8 tsp pepper.
