1. Lightly toast cumin and coriander seeds in a small skillet over medium heat, stirring often, until fragrant, 2-3 minutes; let cool.
2. Purée seeds with jalapeños, scallions, garlic, oil, parsley, cilantro, lemon juice, and salt in a blender or food processor until smooth.

DO AHEAD: Harissa can be made 5 days ahead. Press a piece of plastic wrap directly onto surface of harissa. Cover and chill.