Preheat the oven to 425°F (220°C).

Toss the cubed sweet potatoes with 1 tablespoon of olive oil, salt, and pepper. Spread them on a baking sheet in a single layer.

Roast in the preheated oven for 25-30 minutes, or until tender and slightly caramelized, stirring halfway through.

In a pot, combine the lentils with 3 cups of water. Bring to a boil, then reduce heat and simmer for 20-25 minutes until tender.

Drain any excess water and set aside to cool.

In a mixing bowl, whisk together the olive oil, apple cider vinegar, Dijon mustard, minced garlic, salt, and pepper until well combined.

In a large bowl, combine the roasted sweet potatoes, cooked lentils, mixed greens, cherry tomatoes, and red onion.

Drizzle the dressing over the salad and toss gently to combine. Top with feta cheese if using.

Serve immediately or refrigerate for up to 2 hours before serving.