Heat a small skillet over medium-high heat. Add the cumin seeds and cook, stirring or tossing them frequently, until they are aromatic, 30 seconds to 1 minute. Remove from the skillet and set aside to cool in a small bowl. Repeat with the caraway seeds, and set them aside to cool.

When the cumin seeds have cooled slightly, transfer them to a spice mill or a clean coffee grinder, and process until finely ground. Return the ground cumin to the small bowl and set aside. Repeat with the caraway seeds.

Measure 1 1/2 teaspoons of the ground cumin and 1 teaspoon of the ground caraway, and combine in a mortar. (The remaining ground cumin and caraway can be used for other purposes.)

Place the chiles in the bowl of a food processor, and pulse until chopped. Add the chopped chiles, garlic, salt, and 1 tablespoon of the olive oil to the mortar. Using a pestle, mash repeatedly to form a chunky paste, adding more oil as needed to create a uniform texture. Taste, and adjust the seasoning if necessary by adding more salt or oil. Use immediately, or transfer the harissa to an airtight container, top it with a thin film of olive oil, and refrigerate for several weeks; or freeze in small batches for up to 6 months.