In a small baking dish, combine 1 tbsp. olive oil, 1 tbsp. lemon juice, 2 tsp. cumin and half the garlic. Add the steak and turn to coat. Cover and refrigerate at least 4 hours of overnight.
Preheat the oven to 400 degrees. On a baking sheet, toss the pitas with 3 tbsp. olive oil and the remaining garlic; season with salt and pepper. Bake, tossing once, until golden and crisply, 6-10 minutes. Let cool.
Heat a grill or grill pan over high. Remove the steak from the marinade; season and grill until browned and cooked to desired doneness, about 4 minutes per side for medium-rare. Transfer to a cutting board; let rest 10 minutes.
In a small bowl, whisk the yogurt with the remaining 1 tbsp. olive oil, 2 tbsp. lemon juice and 1 tsp. cumin; season dressing with salt.
In a large bowl, toss the pita croutons, spinach, radishes, cucumber, tomatoes and olives; season and mound on a platter. Thinly slice the steak against the grain and arrange on top of the salad. Drizzle with the dressing.