The canned pineapple syrup is mainly for flavor
Canned pineapple has already been heated during processing, which greatly reduces the activity of the natural enzymes associated with fresh pineapple.
Use the reserved syrup here to carry pineapple flavor into the chicken and sauce rather than relying on it as a strong tenderizer.
Drain the chicken before searing
Let the excess syrup drip from the chicken before it goes into the hot pan. A wet surface cools the pan and makes the chicken more likely to steam.
Keep the reserved marinade because it will be cooked thoroughly as part of the sauce.
Brown the vegetables without cooking them through
The carrots and potatoes only need light color during the initial frying. They will finish cooking later in the sauce.
If they become completely tender during frying, they may break apart while the chicken simmers.
Give the chicken room in the pan
Sear the chicken in batches when necessary. Crowding releases too much moisture at once and makes browning difficult.
Allow each piece enough contact with the hot pan before turning it.
Keep the coconut sauce gentle
Once the coconut milk or cream is added, maintain a gentle simmer rather than a hard boil. High heat can make the sauce separate and become oily.
Stir occasionally and scrape the bottom gently to prevent sticking.
Add the pineapple in two stages
Add half with the coconut milk so it has time to soften and release more flavor into the sauce.
Add the remaining half near the end when you want distinct pieces with a firmer texture. You can adjust this split depending on whether you prefer a smoother or chunkier sauce.
Let residual heat soften the bell peppers
Bell peppers lose their bright color and crispness when simmered for too long. Stir them in after turning off the heat and let the hot sauce soften them slightly.
Cook them briefly before turning off the heat only when you prefer a softer texture.
Make the final seasoning adjustment last
The sauce becomes saltier as it reduces, and the fish sauce and bouillon cube already contribute seasoning.
Taste after the chicken and vegetables are fully cooked, then add only as much salt, fish sauce, or pepper as needed.