Heat olive oil over medium heat in large heavy-bottomed pot and melt butter. Add cubed chicken, season with salt and pepper, and cook for 6 to 8 minutes until golden brown. Remove and set aside.
In same pot, add onion, carrots, celery, and fennel. Season with ½ teaspoon salt and cook for 8 to 10 minutes until softened.
Stir in garlic, rosemary, and thyme. Cook for 1 minute until fragrant.
Pour in chicken stock and turn up heat to bring to boil. Add tortellini and cooked chicken. Lower heat and simmer for 2 to 3 minutes until tortellini is tender.
Turn off heat completely. Stir in pesto, Parmesan, and basil leaves. This preserves the bright pesto flavor.
Adjust seasoning with salt and pepper to taste. Serve hot with extra Parmesan and fresh basil on top.
Notes
Make vegetarian by omitting chicken and using vegetable broth. Add white beans or chickpeas for protein.
Substitute dried herbs (1 teaspoon each) if fresh herbs unavailable.
Deglaze pot with ½ cup white wine after sautéing vegetables for richer flavor.
Store in airtight container in refrigerator for up to 3 days. Tortellini absorbs liquid during storage.
Freeze soup without tortellini and pesto for up to 3 months. Add fresh when reheating.