In a large skillet, combine coconut milk, chicken broth, fish sauce, lime juice, grated ginger, minced garlic, optional sliced chilies, brown sugar, and lime zest. Stir well over medium heat until the mixture begins to gently simmer, about 5 minutes.

Carefully add cod fillets into the simmering poaching liquid, ensuring they are submerged. Reduce heat to low and cover the skillet. Poach fish for 8-10 minutes, or until cod flakes easily with a fork.

In the final minutes of cooking, uncover skillet and stir in spinach leaves. Allow spinach to wilt into broth for about 2 minutes.

Once cod is cooked and spinach is wilted, gently transfer the poached cod and spinach to serving plates. Garnish with fresh cilantro and serve immediately, accompanied by lime wedges.