Preheat the oven to 425°F. Pierce the sweet potato several times with the tines of a fork. Bake it (whole) on a lined baking sheet until the tip of a knife penetrates the skin easily, about 45-50 minutes.
Meanwhile, heat a small amount of water in a frying pan or sauté pan, and then add the diced onions. Sauté for 5 minutes, until the onions soften.
Add the minced garlic and sauté for 3 minutes until fragrant.
Next add the broccoli florets to give them a head start over the more tender vegetables. After a few minutes, add the diced bell pepper. Finally add the sliced mushrooms, and cook for a total of about 10 minutes over low heat, stirring occasionally, until the vegetables are tender.
Turn off the heat and stir in the spinach and ground flaxseed.
Cut the sweet potato in half and lightly mash it with a fork.
Spoon the sautéed vegetables on top each potato half, and enjoy!