Preheat oven to 350°F. Spread pecans in a single layer on an ungreased baking sheet and bake for 8 to 10 minutes until fragrant and tan in the center. Monitor closely during the final minutes to prevent burning. Transfer to a cutting board and roughly chop.
Place sliced onions in a bowl and cover with cold water. Let soak while preparing the rest of the salad to mellow their flavor.
Whisk together the vinegar, oil, poppy seeds, honey, mustard, salt, and pepper in a small bowl or measuring cup until well combined, or shake the ingredients in a sealed mason jar.
Place spinach in a large serving bowl and add the strawberries.
Drain the red onions and add them to the bowl. Drizzle with half the dressing and toss to coat. Add more dressing as needed until the leaves are moistened but not oversaturated.
Add the feta and pecans. Toss lightly to combine and serve immediately with extra dressing on the side.
