In a clean jar with a lid, add the olive oil, balsamic vinegar, honey, vanilla extract, poppy seeds, and black pepper. Close the lid and shake the jar to combine the salad sauce, then place to one side.
Hull and halve the strawberries.
In a large serving bowl, add the spinach, the strawberries, the crumbled feta, and the hazelnuts. Drizzle with the salad sauce just before serving.