Light a grill. Grill the tomatoes and jalapeño over high heat, turning, until charred, 4 minutes. Halve the jalapeño and discard the stem and seeds. Coarsely chop the tomatoes.
Transfer the jalapeño and tomatoes to a food processor. Add the onion, garlic, parsley and cilantro and process until smooth. Season with salt and pepper and let stand for 10 minutes before serving.