Preheat the oven to 425 F (220 C).
Quarter the red peppers, clean out the seeds, and roast them, skin-side up, on a parchment-lined baking sheet. Roast for 20 minutes until the peppers are soft and the skins begin to char.
Remove the peppers, place them in a shallow dish, and cover with plastic wrap or a plate over the top. This makes peeling the skins easier.
Meanwhile, toast the coriander, cumin, caraway seeds, and red chili flakes in a skillet for about 1 minute. Stir them constantly as they toast. Immediately transfer them to a spice grinder or mortar. Add the sumac and grind the spices in or use a pestle to break them apart.
Peel the skins from the roasted red peppers.
If making harissa in a food processor, add the pepper, ground spices, and the rest of the ingredients. Blend until smooth or chunky, depending on your preference.
If making harissa in a mortar, start adding the harder to softer ingredients. Pound as you add ingredients until you have a smooth paste.
Next, remove the skins of the roasted red peppers and dice the flesh.
Makes 2 cups of red pepper harissa paste.