Preheat your oven to 400°F (200°C).
Toss sweet potato cubes with olive oil, salt, and pepper.
Spread sweet potatoes on a baking sheet in a single layer.
Roast for 20-25 minutes until tender.
While the potatoes roast, wash and prepare the mixed greens.
In a small bowl, whisk together olive oil, vinegar, Dijon mustard, salt, and pepper for the dressing.
In a large bowl, combine roasted sweet potatoes, greens, cherry tomatoes, and red onion.
Drizzle the dressing over the salad and toss gently to combine.
Sprinkle feta cheese and walnuts on top.
Serve immediately or chill for later.