Brush skillet with sesame oil, or if you prefer not to use oil, cover bottom of skillet with 1/2 inch of water and bring to a boil. Add onion and sauté for a few minutes. Crumble tofu into skillet and mix well. Cover and cook for 10 minutes, then add other vegetables and tamari. Gently mix, cover, and simmer 3 to 5 minutes.