Rinse the millet under cold water using a fine-mesh sieve until the water runs clear. Set aside.

Heat the cooking oil in a large skillet or pot over medium-high heat.

Add the diced chicken and cook, stirring occasionally, until browned on all sides, about 5-7 minutes.

If using, add the chopped onion and cook until softened, about 3-4 minutes.

If using, add the minced garlic and cook for 30 seconds until fragrant.

Stir in the rinsed millet, broth (or water), salt, and pepper. Bring the mixture to a boil.

Reduce the heat to low, cover the skillet, and simmer for 15 minutes.

Stir in the frozen vegetables. If using, add the soy sauce now.

Cover and continue to simmer until the millet is tender and the liquid is mostly absorbed, and the vegetables are cooked through, about another 10-15 minutes. Stir occasionally to prevent sticking.

Remove from heat and let it rest, covered, for 5 minutes.

Fluff with a fork. Taste and adjust seasoning if needed.

Serve hot. If using, sprinkle with grated cheese before serving.