A blender is the best tool for achieving the smooth texture of this sauce. To dial down the spiciness, remove the seeds from the chiles before blending. Serve over grilled or roasted chicken, pork, beef, or lamb, or spoon it on eggs.
Place packed fresh cilantro leaves, packed fresh cilantro stems, packed fresh flat-leaf parsley leaves, sliced unseeded serrano chiles, fresh lemon juice, garlic clove, kosher salt, ground cumin, ground coriander, and ground cardamom in a blender. Pulse until finely chopped, about 7 times. With blender running, gradually add canola oil until smooth. Serve immediately.