For the Onion Puree:
In a medium saucepan, heat the oil over medium-low heat. Add the onions and cook, stirring often, until they are completely soft but have not taken on any color, 15 to 20 minutes. A bit of water helps here. Add the vinegar and cook for 5 more minutes. Season with salt and pepper. Transfer to a blender and process until satiny-smooth. The puree can be cooled, covered, and stored in the refrigerator for up to 2 days.
For the Cauliflower with Quinoa, Raisins & Peanuts:
Preheat the oven to 400°F.
In a small saucepan, heat 1 table-spoon of the oil over medium-low heat. Add the onion, shallot, and garlic and cook, stirring often, until the onion is softened, about 5 minutes. Add the quinoa and toast, stirring constantly, for about a minute.
Increase the heat to high, add 1 cup of the broth and the thyme, season with salt and pepper, and bring to a simmer. Cook, covered, until almost all the liquid has been absorbed, about 15 minutes. Remove from the heat and let stand for 5 minutes. Discard the thyme sprig.
Trim the stem of the cauliflower so that the head sits flat on a cutting board. Cut down through the center of the head, making 4 thick slices. Most of the slices will still have a bit of stem still attached and that's good.
Heat 2 tablespoons of the olive oil over medium-high heat in the largest skillet you have. Working in batches, brown the cauliflower slices on both sides, about 3 minutes per side. Transfer to a baking pan.
Season the cauliflower with salt and pepper, transfer to the oven, and roast until tender, about 15 minutes.
Meanwhile, add the raisins, chopped peanuts, and onion puree to the quinoa and gently heat through, stirring. Add the remaining 1 cup broth to moisten the quinoa, then stir in the butter and chopped cilantro. Season with salt and pepper.
Lay the cauliflower slices on plates and spoon the quinoa on top. Sprinkle with crushed peanuts, chives and cilantro leaves.