Place spinach and red pepper on large serving platter or divide between four plates.
Heat 1 tablespoon oil over medium-high heat in large skillet. Sauté steak quickly until medium rare. Remove from pan.
Add remaining oil to skillet. Sauté onion slices until softened and translucent. Stir in Marie’s Spinach Salad Dressing, salt and pepper.
Bring to a boil. Drain reserved meat; quickly stir meat into onions. Spoon over spinach; serve immediately.