Rinse quinoa under cold water using a fine-mesh strainer.
In a small saucepan, combine quinoa and water. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes or until liquid is absorbed.
Season the steak on both sides with salt and black pepper.
Heat olive oil in a heavy skillet over medium-high heat until shimmering.
Place steak in the skillet and sear for 4 minutes per side for medium-rare, or until internal temperature reaches 135°F (57°C).
Remove steak from the skillet and allow it to rest on a cutting board for 5 minutes before slicing against the grain into thin strips.
Fluff the cooked quinoa with a fork and divide it between two serving bowls.
Layer baby spinach, cherry tomatoes, cucumber, and red onion over the quinoa.
Top each bowl with the sliced steak and drizzle with balsamic vinaigrette before serving.