Heat 1 tablespoon oil in a large skillet or wok over high heat until shimmering; add chicken pieces in a single layer.
Cook chicken without stirring for 2 minutes, then stir and cook 2–3 minutes more until golden and cooked through; remove to a plate.
Add remaining oil to the same pan; add diced onion and bell pepper and cook about 3 minutes until softened.
Add minced garlic and stir constantly for 30 seconds until fragrant.
Add day-old brown rice, breaking up clumps; press rice against the hot pan briefly to develop some toasted edges.
Return cooked chicken to the pan, add frozen vegetables and low-sodium soy sauce, and toss everything together for about 2 minutes.
Remove from heat and drizzle a little sesame oil; toss one final time. Season with salt and pepper to taste.
Spread the fried rice on a sheet pan and cool completely for about 30 minutes.
Portion into 5 meal prep containers, seal, label with the date, and refrigerate up to 5 days.