Make the dressing: Put the oil, vinegar, honey, mustard, egg, 1/2 teaspoon salt and 1/4 teaspoon pepper in a blender and blend until smooth. Stir in the poppy seeds. Refrigerate until ready to serve.
Make the salad: Put the strawberries in a nonreactive heatproof bowl. In a small saucepan over medium heat, combine the vinegar, 1/2 cup water, the bay leaf, sugar, 1 teaspoon salt and 1/4 teaspoon pepper. Bring to a low boil and allow the sugar to dissolve. Pour over the strawberries and let stand about 20 minutes. Remove the bay leaf and drain the strawberries, discarding the liquid.
In a large salad bowl, combine the spinach, red onion, mushrooms and pickled strawberries. Pour the dressing over the salad, toss well and serve immediately.