Preheat the oven to 350°F (175°C).
Heat the avocado oil in a large heavy-bottomed skillet or pot over medium-high heat.
Add the chopped onion and sauté, stirring occasionally, until translucent, about 5 to 8 minutes.
Stir in the minced garlic and cook for about 2 minutes until fragrant.
Add the coconut milk, chicken broth, vinegar, drained sun-dried tomatoes, chopped kalamata olives (if using), Italian seasoning, and 3/4 tsp sea salt; bring the mixture to a full boil.
Add the baby spinach to the bubbling sauce, cover the skillet for 1 to 3 minutes to wilt, then stir the spinach into the sauce.
Whisk in 1 to 2 Tbsp tapioca flour or gluten-free flour a little at a time until no clumps remain, then continue boiling and stirring frequently until the sauce has thickened, about 5 to 10 minutes; taste and adjust salt or vinegar as needed.
Place the chicken breasts in a large casserole dish and lightly sprinkle them with additional sea salt.
Pour the sun-dried tomato sauce over the chicken, stirring gently so the breasts are well coated.
Bake in the preheated oven for 30 to 40 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and sprinkle with chopped fresh basil before serving.
