Bring 3 cups water to a boil, add rice, and cook according to package instructions.
Combine soy sauce, mirin, and brown sugar in a small saucepan and simmer. Dissolve cornstarch in water, add to sauce, and stir until thickened. Remove from heat.
Heat oil in a large skillet, cook onion and garlic until translucent. Add ground turkey and cook until no longer pink.
Add broccoli, peas, and carrots; cook until vegetables are tender. Pour sauce over mixture and cook 1-2 minutes.
Serve the turkey and vegetable mixture over cooked rice in bowls.
Notes
For extra flavor, garnish with sesame seeds or chopped green onions.