Preheat the oven to 350°F (175°C).
Heat 1 tablespoon avocado oil in a large thick-bottomed skillet or pot over medium-high heat.
Add the finely chopped onion and sauté, stirring occasionally, until translucent, about 5 to 8 minutes.
Stir in the minced garlic and cook for 1 to 2 minutes until fragrant.
Add the coconut milk, chicken broth, vinegar, drained sun-dried tomatoes, chopped kalamata olives (if using), Italian seasoning, and sea salt and bring to a full boil.
Add the baby spinach to the pot, cover for 1 to 3 minutes to wilt, then stir it into the sauce.
Whisk in 1 to 2 tablespoons tapioca flour (or gluten-free flour), breaking up any clumps, then continue boiling and stirring frequently until the sauce thickens, about 5 to 10 minutes. Adjust salt or vinegar to taste.
Place the raw chicken breasts in a large casserole or baking dish and sprinkle lightly with sea salt.
Pour the prepared sun-dried tomato sauce evenly over the chicken, turning the breasts once so they are 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, sprinkle with chopped fresh basil, and serve with desired sides.