Wash and dry the spinach thoroughly to ensure crispness.

Thinly slice the red onion and segment the mandarin oranges.

In a large bowl, combine the spinach, mandarin oranges, and red onion.

Sprinkle slivered almonds and crumbled feta cheese over the salad.

In a small bowl, whisk together olive oil and balsamic vinegar; drizzle over the salad before serving.

Gently toss all ingredients until evenly coated with dressing; serve immediately.