Mix the vinegar, sugar, mustard, salt, onions and poppy seeds together until a thick dressing is formed. If you want, you can do it with the hand blender. Mix the dressing with the washed spinach leaves. Finally, add chopped strawberries and sprinkle with the cheese and nuts.
Note:
The quantity cup corresponds to the American cup. There are also plastic cup sets for sale in Germany, which then contain several copies (including 3/4 cup, 1/2 cup, etc.). I think you can alternatively just take a coffee cup and then pay attention to the proportions of the quantities.