Prep the chicken. Pat the chicken dry and season both sides with salt, pepper, and 1 teaspoon Italian seasoning.
Cut into bite-size pieces for faster cooking, or leave whole to shred later.
Brown the chicken. Heat a large pot or Dutch oven over medium-high heat with 1 tablespoon olive oil and 1 teaspoon reserved sun-dried tomato oil. Sear chicken until lightly browned (3–4 minutes per side if whole, or 4–6 minutes total if cubed). Transfer to a plate.
It’s fine if it’s not fully cooked yet.
Sauté the aromatics. In the same pot, reduce heat to medium. Add a splash more oil if needed. Cook chopped onion with a pinch of salt until soft and translucent, 4–5 minutes.
Stir in minced garlic and tomato paste; cook 1 minute until fragrant.
Build the base. Stir in sun-dried tomatoes, remaining Italian seasoning, and red pepper flakes. Pour in chicken broth, scraping up any browned bits from the bottom of the pot for extra flavor.
Simmer the chicken. Return chicken to the pot. If using whole pieces, simmer gently for 12–15 minutes until cooked through, then remove and shred.
If using bite-size pieces, simmer 8–10 minutes until cooked. Keep the heat at a steady simmer, not a rapid boil.
Add the greens. Stir in the spinach and cook until wilted, about 1–2 minutes. Turn heat to low.
Make it creamy without curdling. In a bowl, whisk the Greek yogurt with a ladle of hot soup until smooth and warm (this tempers it).
Stir the yogurt mixture back into the pot slowly. Add Parmesan and stir until melted. Keep the heat low to avoid splitting.
Adjust thickness. If you want a thicker soup, whisk 1 tablespoon cornstarch with 2 tablespoons cool water, then stir into the soup and simmer 2–3 minutes.
Repeat with another tablespoon if needed.
Finish and season. Add a squeeze of lemon juice for brightness, if you like. Taste and adjust salt, pepper, and chili flakes. The flavor should be savory, a little tangy, and gently spicy.
Serve. Ladle into bowls and top with extra Parmesan and fresh basil or parsley.
A slice of crusty bread or a side salad pairs nicely.