Pat the chicken breasts dry and season both sides with sea salt.
Heat 2 to 3 tablespoons avocado oil in a large nonstick skillet over high heat until very hot.
Sear the chicken breasts in a single layer for about 2 minutes per side, working in batches if needed, until golden brown; transfer to the crock pot.
Reduce heat to medium or medium-high and sauté the sliced onion for 2–3 minutes until starting to soften, then add minced garlic and cook 1–2 minutes more.
Add the sautéed onions and garlic to the crock pot with the browned chicken, then add the chopped carrots.
In a bowl or measuring cup, whisk together the coconut milk, fish sauce, lime juice, maple syrup, gluten-free flour (if using), and salt until combined.
Pour the coconut-lime sauce over the chicken and vegetables in the crock pot and stir gently to combine.
Cover and cook on high for 2 hours.
After 2 hours, uncover and add the chopped zucchini, then recover and continue cooking for another 1–2 hours until chicken is fork-tender and vegetables reach desired doneness (about 3 hours total).
Remove the lid, stir in the baby spinach and replace the lid for a few minutes until the spinach wilts, then stir in the chopped basil before serving.
