Heat 1 tablespoon oil in a large frying pan set over medium heat. Add onion, carrots and garlic. Sauté, stirring occasionally, until onion is softened, about 5 minutes. Stir in jalapeño, pepper and peas. Sauté for another 2 minutes. Remove from heat and add to lentils.
In a bowl, whisk 1 tablespoon oil with lemon juice, cumin, salt and cayenne, if using. Stir into salad. Taste and add more salt, if needed.
If making ahead, cover and refrigerate up to 2 days, then add fresh herbs, if using, just before serving.