To make the cauliflower
Preheat the oven to 230°C/450°F/gas mark 8.
Turn the cauliflower upside down, cut it in half, and then cut those halves in half, and so on, depending on the size. You want pieces around 2 inches wide.
Place the cauliflower in a large bowl, and using a rubber spatula or your hands, toss it with the oil, salt, and pepper until completely coated.
Arrange the cauliflower in a single layer in a baking dish or on a sheet pan. Roast until fork-tender and caramelized, 30 to 35 minutes.
To make the salsa verde
Combine the parsley, garlic, and capers in a stand blender or the small bowl of a food processor, and pulse briefly a few times, as you want a textured sauce. Add 6 tablespoons of the oil, the lemon juice, oregano, and cayenne and pulse a few more times. Taste for acidity and texture, adding more lemon juice as needed. If the sauce is too thick, add the remaining oil.
Serve the cauliflower warm or at room temperature, passing the sauce at the table to spoon over the top. The cauliflower will keep for a few days in an airtight container in the refrigerator. Store any remaining salsa verde separately.
Kitchen notes: Estimate ½ pound cauliflower for each serving. The salsa verde will yield a scant 1 cup and can be made a day in advance.