In a medium saucepan, heat 1 Tbsp oil over medium heat. Add onion, carrot, and celery, and cook, stirring occasionally, until tender, about 6 minutes. Add lentils and cover with 2 inches of water. Bring to a boil, then reduce heat and simmer until lentils are tender, about 20 minutes. Drain and season with salt and pepper. Let cool.
In a medium bowl, whisk together remaining oil, lemon juice, and pepper flakes. Season with salt. Add lentils, almonds (if using), cabbage, and herbs and toss to combine.
Refrigerate lentil salad in an airtight container up to 3 days.