Preheat the oven to 425° Fahrenheit.
Prepare the cauliflower: remove any leaves from the cauliflower and chop into bite-sized florets. In a medium bowl, toss the cauliflower with the oil. Then toss the cauliflower with 1 tablespoon cornstarch.
Bake the cauliflower: add the cauliflower in one layer on a foil or parchment paper lined baking sheet. Bake for 10 minutes, flip and bake for 5 - 10 minutes more, until the cauliflower is browned (it will be dark brown in some spots) and cooked through.
Make the sauce: as the cauliflower cooks, make the sauce. In a small bowl, stir together the maple syrup, ketchup, soy sauce, vinegar and garlic powder.
Cook the sauce: in a small skillet over medium high heat, bring the sauce to a boil. Turn the heat to low and simmer. In a small bowl, mix 1 tablespoon cornstarch with 1 tablespoon of water. Pour this mixture into the sauce and simmer 3 - 5 more minutes, until the sauce has thickened.
Serve: in a large bowl, gently toss the cauliflower in the sauce. Optionally, serve with a side of white rice and topped with green onions & sesame seeds.