Steps
1. In a skillet, heat the olive oil over medium heat.
2. Add the thinly sliced beef and cook until browned, about 4-5 minutes.
3. Stir in the balsamic vinegar, Dijon mustard, salt, and pepper.
4. Remove from heat and mix in the red onion and walnuts (if using).
5. In a bowl, place the fresh spinach and pour the warm beef dressing over it.
6. Toss gently to combine and top with grated Parmesan cheese if desired.
7. Serve immediately while the dressing is warm.