Sauté the vegetables: In a large pot, heat oil over medium heat. Add onion, carrots, and fennel, seasoning with salt and pepper. Sauté for about 8 minutes until softened.

Build the soup base: Stir in balsamic vinegar and garlic, sautéing for 1 minute. Add tomatoes, broth, thyme, and red pepper flakes. Simmer, cover, and cook for 30 minutes.

Cook the tortellini: Boil salted water in a separate pot. Add tortellini, cooking for 3-5 minutes until they float. Drain and set aside.

Combine tortellini and kale: Stir in cooked tortellini and kale into the pot. Simmer for an additional 2 minutes until kale is tender.

Final seasoning and serving: Adjust seasoning with salt and pepper. Serve hot, garnished with kale pesto and chopped herbs.