Whisk the soy sauce, garlic, coconut sugar, water, sriracha and cornstarch together in a small bowl. Set aside.
Heat the oil in a large skillet over medium-high heat. Once hot, add the onions and cook for 2-3 minutes until softened.
Add the sauce to the skillet and let cook for about 1 minute. Add the chicken and zucchini noodles to the skillet, toss to fully coat in the sauce and cook for about 2 minutes, until the zucchini noodles are just slightly softened. Serve immediately.