Heat a medium pan over high heat and add oil. Reduce heat to medium, add sliced mushrooms and vinegar and let them cook fully on one side for about 5 minutes, then flip. When mushrooms are nearly done, add walnuts, toss once and remove from heat. (If you overcook the walnuts, they will taste bitter.).
Divide spinach between two plates and top with mushroom and walnut mixture. add salt and pepper to taste.
Note: for a little extra protein, you can add sauteed shrimp or tofu.