Begin by preparing the homemade vinaigrette. In a small bowl, whisk together the olive oil, apple cider vinegar, Dijon mustard, honey, and minced garlic until well combined—season with salt and pepper to taste. Set aside to let the flavors meld.
Combine the fresh spinach leaves, sliced hard-boiled eggs, chopped crispy bacon, thinly sliced red onion, and sliced mushrooms in a large salad bowl.
Drizzle the prepared vinaigrette over the salad ingredients and toss gently to ensure everything is evenly coated.
Sprinkle the toasted walnuts and crumbled feta cheese over the top of the salad.
Season with additional salt and pepper if needed, and give the salad one final toss before serving.