Heat oven to 425°F. Toss sweet potato cubes with 2 tablespoons olive oil, 1/2 teaspoon salt, and pepper on a rimmed baking sheet. Spread in a single layer and roast for 25-30 minutes, flipping once, until deeply caramelized at the edges and tender when pierced with a fork.

While potatoes roast, combine lentils with 3 cups water and remaining 1/2 teaspoon salt in a medium saucepan. Bring to a boil, then reduce heat and simmer uncovered for 20-25 minutes until tender but still holding their shape. Drain any excess water and let cool slightly.

In a small bowl, whisk together remaining 1 tablespoon olive oil, minced garlic, lemon juice, Dijon mustard, and honey until emulsified. Let sit for 5 minutes to mellow the raw garlic.

In a large bowl, combine warm lentils and roasted sweet potatoes. Pour dressing over top and toss gently to coat. Fold in parsley and feta. Taste and adjust seasoning with additional salt, pepper, or lemon juice as needed. Serve warm or at room temperature.