Step 1

Position a rack in the middle of the oven and preheat to 450 degrees. Line a large sheet pan with foil.

Step 2

Cut off the top of the head of garlic to expose the cloves. Place the head of garlic on a small piece of foil, drizzle with 1 teaspoon of the oil and wrap to seal. Place the wrapped garlic on the pan, and add the bell peppers and red chiles. Transfer to the oven and roast for 15 minutes.

Step 3

Transfer the chiles to a large bowl and cover with a wide plate. Turn the bell peppers over, return to the oven and roast – along with the garlic – for another 15 minutes, or until the peppers are tender, with blistered skins, and the garlic is soft.

Step 4

Transfer the bell peppers and garlic to the bowl with the chiles and re-cover to let the peppers steam so their skins loosen as they cool. Reduce the oven temperature to 425 degrees.

Step 5

In a large bowl, toss the broccolini with 1 tablespoon of the oil and the salt. Transfer the broccolini to the same foil-lined sheet pan and roast for about 15 minutes, or until it is firm-tender with lightly crisped tops, turning once midway through.

Step 6

While the broccolini roasts, arrange the walnuts on a small sheet pan and place in the oven for about 5 minutes, or until fragrant and toasted. Let the walnuts cool, then coarsely chop.

Step 7

Once the bell peppers and garlic are cool enough to handle, about 20 minutes, peel off and discard the pepper skins, then discard the stems. Cut the peppers in half lengthwise and discard the seeds. Squeeze 6 cloves of garlic from the skins.

Step 8

To a food processor, add the bell peppers, 1 of the chiles, the 6 garlic cloves, the olives and capers and pulse until combined. (Refrigerate the remaining garlic cloves for another use.) Taste, and add the remaining 1 chile, if desired. Add the remaining olive oil and pulse to combine. Taste, and season with additional salt, if desired.

Step 9

To serve, spread a heaping 1/4 cup of the sauce onto each plate, top with the broccolini and sprinkle with the walnuts.