In a large skillet, heat the butter over medium/medium-high heat. Add the sliced chicken pieces in a single layer and cook, flipping/stirring occasionally, until fully cooked, about 5-6 minutes. Transfer the cooked chicken to a clean plate and cover with another plate or foil to keep warm.
Pour the orange juice sauce into that same large skillet. Bring to a boil over medium-high heat, then while whisking the sauce with one hand, pour in the arrowroot-water mixture. Continue whisking until fully incorporated - work quickly to avoid the sauce gelling inconsistently.
Add the stir-fry vegetables and cook them in the sauce, stirring frequently, until they are just tender, still with a little crunch, about 4-6 minutes. At this point, the sauce should have thickened quite a bit. Add the chicken and stir to reheat it, about 1 minutes.
Serve the chicken and vegetables spooned on top of the cauliflower rice, sprinkle with the sliced green onions (and chili flakes, if using), and dig in.