In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat until it shimmers.
Add the diced yellow onion to the skillet and sauté for about 2-3 minutes, until translucent. Stir in the minced garlic and grated ginger (if using) for another minute until fragrant.
Toss in the sliced green onions (reserve some for garnish), diced red bell pepper, and frozen peas and carrots mix. Stir-fry for about 3-4 minutes.
Add the chilled brown rice, breaking up any clumps. Stir to mix well with the vegetables.
Pour in the soy sauce or tamari and toasted sesame oil, stirring to coat. Season with salt and black pepper to taste. Cook for an additional 2-3 minutes.
Remove from heat and garnish with reserved green onions. Serve hot and enjoy!