Cook the rice noodles according to package instructions; drain, set aside, and keep warm.
Heat the 2 Tbsp avocado oil in a large stockpot over medium-high heat.
Add the 1/2 medium finely chopped onion and sauté 5–8 minutes, stirring occasionally, until translucent.
Add the 3 cloves minced garlic and the 3 lbs chopped chicken breasts. Cook 3–4 minutes, stirring, until the chicken is lightly browned on the outside but not fully cooked through.
Add both cans of full‑fat coconut milk and the entire jar of Green Curry Paste to the pot. Stir well to combine and dissolve the paste into the coconut milk.
Add the chopped broccoli crown, chopped carrots, red bell pepper match sticks, and chopped zucchini squash. If using, add the 2 tsp fish sauce and 1/4 tsp red pepper flakes now.
Bring the mixture to a boil, then reduce heat to a simmer. Cover and cook about 10 minutes, or until the chicken is cooked through (165°F/74°C) and the vegetables are tender.
Stir in the cooked rice noodles and heat 1–2 minutes until the noodles are warmed through. Serve immediately.