Begin by washing the Honeycrisp apples under cold water. Slice them thinly, keeping the skin on. Thinly slice half a red onion and place in ice water for 10 minutes. Wash a large handful of baby spinach leaves.

In a dry skillet over medium heat, toast the walnuts for 2–3 minutes, shaking the pan occasionally. Set aside to cool.

In a small bowl, whisk together olive oil, apple cider vinegar, honey, and Dijon mustard. Season with salt and pepper to taste.

In a large bowl, combine the sliced apples, crumbled feta, toasted walnuts, and spinach. Toss gently.

Pour the vinaigrette over the salad and toss gently to coat. Serve immediately.