Prep the onion, carrots (slice them very thin so they cook fast), and bell pepper.
Add the oil and onion to a skillet over medium-high heat. Sauté the onion for 5 minutes.
Meanwhile, cut up the chicken into bite size (about 1") pieces and season them with salt & pepper (just doing this on the cutting board is fine).
Add in the chicken and cook it, stirring occasionally, for 5 minutes. Transfer chicken to a plate and set aside.
Stir in the garlic and curry paste and cook for 30 seconds.
Add the fish sauce, coconut milk, sugar, sliced carrot, and peppers to the skillet. Let it come to a boil and then reduce the heat and simmer for 6 minutes or so.
Add the chicken back to the skillet and cook it for a few more minutes, letting it heat through (but don't overcook).
Add the lime juice (start with my suggestion and add more to taste if you like) and salt & pepper as needed.
Stir in the cilantro, basil, and scallions just prior to serving (use a clean cutting board to chop them!). Serve immediately over cooked rice or noodles.