Heat the avocado oil in a large pot or Dutch oven over medium heat until shimmering.
Add the diced onion and cook, stirring occasionally, for 3 to 4 minutes until it begins to soften.
Stir in the yellow curry paste (or curry powder), minced garlic, and grated ginger; cook for 1 to 2 minutes until fragrant.
Add the chopped carrots and red bell pepper, cover, and cook 3 to 4 minutes until they begin to soften.
Stir in the chopped chicken and zucchini so they are evenly distributed in the pot.
Pour in the coconut milk, chicken broth, maple syrup (or sugar), fish sauce if using, and sea salt; bring to a full boil.
Reduce heat to medium and simmer, stirring occasionally, for about 20 minutes until the chicken is cooked through and the vegetables are tender.
Stir in the Thai basil just before serving and ladle the soup into bowls.