Heat olive oil in a skillet over medium-high heat. Sauté carrots and onion for 4-5 minutes. Add garlic and cook for 20 seconds. Season with red pepper flakes, salt, and pepper.
Transfer vegetable mixture to a 6-quart crockpot. Add lentils, broth, kielbasa, and salt. Stir and cover. Cook on low for 5.5 hours.
Stir in Italian herbs and lemon juice. Adjust seasoning as needed.