In a large skillet, heat olive oil over medium heat. Add the onion and garlic, sautéing until soft and fragrant.
Stir in the cooked chicken, taco seasoning, and diced tomatoes with green chilies. Cook for 2–3 minutes to blend the flavors.
Add the tortellini, corn, black beans, and chicken broth. Stir well and bring to a simmer. Cover and cook for 5–6 minutes, until the tortellini is tender.
Lower the heat and mix in the sour cream and cheddar cheese. Stir until the cheese melts and the sauce becomes creamy.
Taste and adjust salt and pepper. Garnish with chopped cilantro or green onions if desired.