In a saucepan, bring the water for the millet to a boil. Add the millet, lower the heat to low, cover, and simmer until tender, about 25 minutes.
Remove from the heat and set aside. Preheat the oven to 350°F (175°C).
In a large skillet, heat a small amount of water, as needed, over medium heat. Add the red onion and cook for 5 minutes. Stir in the kale and cook until tender, about 5 more minutes. Drain off any remaining liquid and set aside to cool.
Blend together the white miso paste and hot water.
Add the millet to the kale mixture in the skillet. Stir in the miso mixture, drained barberries, sunflower seeds, flaxseeds, nutritional yeast, pippali, and sodium-free salt substitute (if using). Mix well. Taste to adjust the seasonings, if needed.
Tightly pack the bell pepper halves with the millet stuffing and arrange them in a large shallow baking dish. Add about ½ cup (120ml) of water to the baking dish, cover tightly, and bake until the peppers are tender and the stuffing is hot, 30 to 40 minutes. Serve hot, two pepper halves per serving.