Heat the oil in a large pot over medium heat. Add the onion, carrots, fennel, salt, and a few grinds of pepper and cook, stirring, until the vegetables begin to soften, about 8 minutes.

Add the balsamic vinegar, garlic, tomatoes, broth, thyme, and red pepper flakes. Cover and simmer for 30 minutes, or until the vegetables are tender.

Meanwhile, cook the tortellini in a pot of salted boiling water according to the package directions until al dente. Add the tortellini and the kale to the soup and simmer for 2 more minutes.

Season to taste with about 1/4 to 1/2 teaspoon more salt and a few generous grinds of pepper.

Serve in bowls with scoops of kale pesto and fresh parsley for garnish.