Whisk together the fish sauce, brown sugar, red pepper flakes, chili paste, and lime juice and zest in a small bowl. Put aside to dissolve the sugar.
Heat peanut oil in wok or large skillet over medium-high heat until nearly smoking. Add chicken cubes and cook, stirring occasionally, until juices run clear and some browning occurs, 5-6 mins. Remove chicken from wok, put onto warm plate and cover with foil.
Stir fish sauce mixture and pour into wok, with heat on medium-high. Bring sauce to boil, then let boil for a 2-3 minutes, until it starts to thicken [?reduce].
Add carrots and onions and cook, stirring frequently, until they begin to soften (2-3 mins).
Add bell pepper and cook for a minute or so. Push veggies to the side, leaving sauce in the center, add bean sprouts to sauce, and cook sprouts, turning frequently, 1 minute.
Push all veggies back into center, add chicken and stir all ingredients together. Let heat for a minute or two, stirring occasionally.
Remove from heat and transfer to serving dish. Garnish with chopped cilantro and lime wedge, and chopped peanuts, if available. Enjoy!