Wash and dry spinach. Place the leaves in a large salad bowl; set aside.
Whisk together 2 tablespoons olive oil, the garlic, lime juice, sugar, and soy sauce in a small bowl; set aside.
Heat 1 tablespoon olive oil in a cast-iron skillet or grill pan on prepared grill. When oil is hot but not smoking, sauté onion, stirring constantly, for 3 minutes. Transfer onion to bowl with spinach. Add remaining 1 tablespoon olive oil to skillet. Sear steak over medium-high heat about 3 minutes per side or until medium-rare. Remove skillet from heat, and immediately transfer the steak to a cutting board; let rest.
Return skillet to heat, and add soy-lime mixture to pan; turn heat to medium-high. Deglaze skillet by stirring constantly and loosening brown bits on bottom. Cover and keep warm.
Slice steak as thinly as possible, cutting against grain, and arrange over onions and greens. Drizzle with sauce, and garnish with a cilantro sprig. Sprinkle with peanuts (if desired). Serve immediately.