Wash the baby spinach thoroughly under cold water. Drain and pat dry with a towel.
In a large salad bowl, add the spinach, cherry tomatoes, sliced red onion, crumbled feta cheese, and sliced almonds.
In a small bowl, whisk together the olive oil, balsamic vinegar, Dijon mustard, salt, and pepper until well combined.
Drizzle the dressing over the salad ingredients in the large bowl.
Toss gently with salad tongs until the salad is well coated with the dressing.
Serve immediately, dividing the salad into four bowls.
For customization, consider adding toppings such as avocado, grilled chicken, or hard-boiled eggs.
If making the dish vegan, omit the feta cheese or use a plant-based alternative.
Store any leftovers in an airtight container in the refrigerator for up to 2 days without dressing.