Step 1
Combine the chicken stock, white wine, and tomato paste in a 6- to 8-quart slow cooker, whisking until the paste is fully dissolved. Add the crushed tomatoes, balsamic vinegar, garlic, oregano, onion powder, red pepper flakes, and kosher salt. If using unsalted stock, incorporate an extra ½ teaspoon of salt. Add the chicken thighs and stir to ensure even distribution of the ingredients. Cook on low heat for 5 to 6 hours, or until the chicken becomes exceptionally tender and the flavors meld together.
Step 2
Raise the heat setting to high for approximately 10 minutes. Using two forks, shred the chicken directly in the slow cooker by pressing the thighs against the sides of the pot. Add the baby spinach, stirring gently to wilt it into the soup.
Step 3
Incorporate the tortellini, cover the slow cooker, and let it cook until the pasta reaches an al dente texture and is thoroughly warmed, about 3 minutes. Adjust the seasoning with additional salt, if necessary. Serve the soup hot, garnished with Parmesan cheese, fresh basil, and a sprinkling of black pepper.