Wash the Honeycrisp apples thoroughly under cool water. Slice them into thin wedges, removing the core, and place them in a large mixing bowl. Soak thinly sliced red onion in cold water while you prep the other ingredients. Wash and dry the baby spinach.

In a sauté pan over medium heat, add pecans or walnuts and toast for 2-3 minutes until fragrant and lightly browned. Remove from heat and let them cool.

In a small mixing bowl, combine olive oil, apple cider vinegar, honey, and Dijon mustard. Whisk vigorously for about 30 seconds until emulsified and creamy. Season with salt and pepper to taste.

In a large serving bowl, combine sliced Honeycrisp apples, baby spinach, crumbled feta, and toasted pecans. Toss lightly to mix well while preserving texture.

Drizzle the vinaigrette over the salad just before serving. Toss carefully to combine flavors without bruising the fruit.