Sauté the vegetables: Heat olive oil in a large pot over medium heat. Add the onion, carrots, fennel, salt, and black pepper. Cook, stirring occasionally, until the vegetables soften, about 8 minutes.

Add the flavorings: Stir in balsamic vinegar, garlic, diced tomatoes, vegetable broth, thyme, and red pepper flakes. Cover and simmer for 30 minutes, until the vegetables are tender.

Cook the tortellini: While the soup simmers, cook the tortellini in salted boiling water according to package directions until al dente. Drain and set aside.

Combine and finish: Add the cooked tortellini and kale to the soup. Simmer for 2 more minutes until the kale wilts.

Season and serve: Taste and adjust seasoning with more salt and pepper if needed. Serve in bowls with a generous scoop of kale pesto and a sprinkle of fresh parsley or basil.