Position a rack in the middle of the oven and preheat to 450 degrees. Line a large, rimmed baking sheet with parchment paper.
Pull off or cut away the large leaves from the cauliflower, saving them for soup or broth, if you’d like. Trim a slice from the core end, where it’s likely discolored. While holding the cauliflower at a comfortable angle, curved side down, insert your knife tip into the core. As you push the knife in farther, gently rock the blade side to side and back and forth. The head should crack and naturally break into two halves.
Cut each half into four wedges.
In a large bowl, stir together the hoisin sauce, sesame oil, agave, ketchup, soy sauce, garlic and five-spice powder. Add the cauliflower wedges and, using a big spoon or spatula, gently toss and stir to coat well. Spread the wedges out onto the baking sheet, cut side down. Smear any remaining seasoning from the bowl onto the wedges.
Roast the cauliflower for 15 minutes, use tongs to turn over the wedges, and roast for another 10 minutes. Liquid will appear on the pan. Continue roasting for another 5 to 10 minutes, flipping the cauliflower pieces once or twice more, as the liquid concentrates, bubbles and thickens, until the cauliflower is richly browned and tender but slightly chewy.
Serve hot or at room temperature.
