Preheat the oven to 425°F.
Cut the cauliflower into large bite size florets. In a medium bowl toss the cauliflower with 1 ½ tablespoons oil.
Place the cauliflower in one even layer on a rimmed baking sheet and bake for 15 minutes, (Add an additional 5 minutes if desired. I like mine to be tender, but still firm). Cauliflower should be just browning on the edges.
While the cauliflower is roasting, whisk together the orange juice, soy sauce, maple syrup, and vinegar. Set aside.
In a separate small bowl, whisk together the cornstarch and water until smooth and set aside.
Add the remaining 1 ½ tablespoons oil to a large non-stick skillet. Add the onion and sauté 4 to 5 minutes until tender.
Add the garlic and peppers. Sauté 3 to 4 minutes until starting to soften.
Add the orange sauce to the pan and turn up the heat to bring to a boil.
Once boiling, add the cornstarch slurry and stir well to incorporate. Turn the heat down to medium-low and simmer until the sauce reduces and thickens to a syrup like consistency, about 10 minutes or so.
Add the roasted cauliflower to the sauce stir to coat.
Sprinkle sesame seeds over the top, if using, and serve immediately by itself or over rice.