Place the strawberries on a cutting board and hull them using a sharp knife, then slice into thin pieces. Set aside.
Transfer the washed spinach into a salad spinner and spin until completely dry. Tear or chop into bite-sized pieces and place in a large mixing bowl.
Add the sliced strawberries to the bowl of spinach, along with the thinly sliced red onion. Gently toss to combine, ensuring the ingredients are evenly distributed.
In a small bowl, whisk together balsamic vinegar, olive oil, and honey until the dressing is smooth and slightly thickened.
Pour the dressing over the salad and toss gently to coat all the ingredients evenly. The strawberries should release some juice, making the salad look vibrant and glossy.
Sprinkle the crumbled feta cheese over the top of the salad, then give it a final gentle toss to distribute the cheese evenly.
Serve the salad immediately for the freshest texture and brightest flavors, or refrigerate for up to a few hours, adding fresh strawberries just before serving if prepared early.