Toast the pecans: Preheat the oven to 350 degrees F. Spread the pecans in a single layer on an ungreased baking sheet. Bake for 8 to 10 minutes, until the pecans smell fragrant and the center of a pecan is tan when the pecan is broken in half. (Do not walk away from the oven in the last few minutes of cooking. This is when nuts love to burn.) Transfer to a cutting board and roughly chop.
Place the sliced onions in a bowl and cover with cold water. Let sit while you prepare the rest of the salad (this keeps their flavor but removes the harsh onion bite).
Prepare the dressing: In small mixing bowl or large liquid measuring cup, whisk together all of the dressing ingredients - vinegar, oil, poppy seeds, honey, mustard, salt, and pepper - until well combined. (Alternatively, you can shake the ingredients together in a mason jar with a tight-fitting lid).
Assemble the salad: Place the spinach in a great big serving bowl. Add the strawberries. Drain the red onion and add it as well. Drizzle about half of the dressing over the salad and toss to coat the leaves. Assess the amount of dressing. You want the spinach leaves to be nicely moistened but not swimming in dressing. Add a little more if needed to suit your preferences. Add the feta and pecans. Toss lightly to combine. Serve immediately, with extra dressing on the side as desired.