Heat olive oil in a non-stick skillet over medium heat.

Add chopped onion and sliced celery to the skillet.

Cook the onion and celery until crisp-tender, about 3-5 minutes.

Add minced garlic to the skillet and sauté, being careful not to burn it.

Rinse and sort the dried lentils.

Combine the sautéed vegetables and garlic with the remaining ingredients (lentils, frozen vegetables, vegetable broth, diced tomatoes, thyme, salt, pepper, and red pepper flakes) in a large Crock-Pot.

Cook on Low for 4 hours or on High for 2 hours, or until lentils are tender.

If you want a more crisp-tender texture for the frozen vegetables, add them during the last 30-60 minutes of cooking time.

Garnish with shredded mozzarella cheese and finely chopped roasted red peppers before serving.

Alternatively, serve over or stir in your favorite pasta or rice.