Preheat oven to 425°F (220°C). Peel and cube the sweet potatoes.
Toss sweet potatoes in 1 tablespoon of olive oil and season with salt and pepper. Roast on a baking sheet for about 25 minutes, flipping halfway through.
While the sweet potatoes roast, combine lentils with 3 cups of water in a pot. Bring to a boil, then simmer for 20-25 minutes until tender. Drain and cool slightly.
In a large bowl, mix lentils, chopped red onion, and baby spinach.
Whisk together remaining olive oil and balsamic vinegar, then pour over the salad mixture. Toss gently to combine. Serve chilled or at room temperature.
