Heat 2 Tbsp. olive oil in a large skillet over medium. Once hot, add bell pepper, green beans, and red onion. Cook for 7 to 9 minutes, until tender. Season with ¼ tsp. salt and black pepper; transfer to a bowl to slightly cool.
Prepare Lemon-Dijon DressingIn a small bowl, combine olive oil, lemon juice, Dijon mustard, cumin, salt and pepper; whisk to combine. Stir in fresh dill.
Assemble SaladPlace lentils in a large bowl. Add sautéed veggies, dressing, and ½ cup feta; toss well to combine. Garnish with remaining ¼ cup feta and extra fresh dill, if desired. Serve warm, chilled, or closer to room temperature.
