Wash the mixed salad greens thoroughly and pat dry.
Place the greens in a large salad bowl.
Peel and segment the mandarin oranges.
Add the mandarin orange segments to the bowl.
Slice the red onion thinly and scatter over the salad.
Sprinkle the sliced almonds over the top.
If using, add the crumbled feta cheese.
Whisk together olive oil and balsamic vinegar in a small bowl.
Drizzle the dressing over the salad and toss gently.
Season with salt and pepper to taste and garnish with fresh basil.