In a small skillet, toast the cumin and coriander seeds over moderate heat, stirring, until the seeds are lightly browned and fragrant, about 2 minutes.
Put the chiles in a small saucepan, cover with water and bring to a boil. Remove from the heat and let cool. Remove the stems and scrape the seeds from the chiles. In a food processor, combine the anchos, parsley, cilantro, garlic and the toasted cumin and coriander seeds; process until smooth. Add enough of the olive oil to make a very loose paste.
The harissa can be refrigerated, covered, for up to 5 days.