Cut the chicken into medium-sized chunks and season with salt and pepper.
Heat a medium saucepan over medium heat until hot. Swirl in the coconut oil. When the oil is shimmering, brown the chicken and transfer to a separate plate.
Add the onions to the saucepan and sauté until translucent. Return the chicken and juices to the pot, and add the curry paste to taste. (Start with 1 tablespoon and add more if needed later.)
Stir-fry until you can smell the rich aroma of toasted spices. Pour in the coconut milk and toss in the frozen veggies, squash, and apple juice (if using).
Raise the heat to high and bring the contents of the pot to a boil. Then, turn the heat down to medium-low and simmer the curry for 10 minutes or until the veggies are warmed through and the flavors meld. Taste for seasoning and adjust as needed.