This simple dish can easily be thrown together. For your next gathering, make the sauce a few hours ahead and keep it in the fridge until you’re ready to serve. The cauliflower cooks quickly: Toss it in the oven to roast while the main dish rests.
Position 2 oven racks in the upper and lower thirds of the oven and preheat the oven to 450ºF. Line two baking sheets with parchment paper.
Cut the florets off of the cauliflower heads and cut into 1/2-inch pieces; put into a large bowl. Add 1/3 cup of the olive oil and toss to coat. Spread the florets out evenly on the 2 prepared baking sheets, without overlapping, and sprinkle with 1 teaspoon salt and a few grinds of pepper between the two baking sheets. Roast the cauliflower until charred in spots and tender when pierced with a knife, 22 to 24 minutes.
Meanwhile, combine the capers and garlic in a food processor and pulse until finely chopped. Add the parsley, scallions, lemon zest and juice and continue to pulse until the leaves are coarsely chopped. With the machine running, pour in the remaining 1/2 cup olive oil and process until the mixture forms a loose pesto–like consistency. Season with a pinch of salt to taste.
Transfer the cauliflower to a serving platter and the herb-caper sauce to a small serving bowl or ramekin. Serve the cauliflower with dollops of the sauce.