Brush the flank steak with 1 tablespoon of olive oil and season both sides evenly with salt and black pepper.
Preheat a grill or heavy cast-iron skillet over medium-high heat until slightly smoking.
Sear the steak for 5 to 6 minutes per side for medium-rare, or until the internal temperature reaches 135°F (57°C).
Transfer the steak to a cutting board and allow it to rest for a full 10 minutes to retain juices.
While the steak rests, whisk together the Greek yogurt, parsley, dill, minced garlic, lemon juice, and a pinch of salt in a small bowl until smooth.
Divide the cooked quinoa and baby spinach equally into four serving bowls.
Slice the rested steak thinly against the grain to ensure tenderness.
Arrange the sliced steak, cherry tomatoes, and cucumber slices over the quinoa and spinach base.
Top each bowl with a generous dollop of the creamy herb sauce and serve immediately.