Bring a large pot of salted water to a boil and cook the gluten-free pasta according to package instructions, cooking about 2 minutes less than recommended for al dente. Drain and set aside.
Heat the avocado oil in a large skillet over medium-high heat.
Add the finely chopped yellow onion and sauté, stirring occasionally, until softened, about 5 minutes.
Stir in the minced garlic and the chopped chicken. Let the chicken brown on one side for a few minutes, then stir and continue cooking 2–3 minutes more until the pieces are browned but not completely cooked through.
Pour in the full-fat coconut milk, add the lemon zest, lemon juice, and sea salt, and stir to combine.
Bring the sauce to a boil, then reduce to a simmer and cook until the chicken is cooked through and the sauce has thickened slightly, about 5 minutes.
Add the drained pasta to the skillet and toss until the noodles are evenly coated with the sauce.
Taste and adjust seasoning with more sea salt or lemon juice as desired, then stir in the chopped fresh basil if using and serve.