Prepare cilantro sauce. In food processor, mince cilantro and jalapeño very fine. Spoon in crema and salt, process again until well blended. Refrigerate until needed. (Sauce can be made 1 to 2 hours ahead, but after that the cilantro’s vibrancy begins to fade.).
Prepare filling, mixing together all ingredients in medium bowl. Cover and chill until needed. (Filling can be combined 1 day in advance, covered, and refrigerated.).
Fire up grill, bring temperature to medium heat (4 to 5 seconds with hand test).
Grill chiles uncovered 8 to 10 minutes, turning occasionally so that skin blackens and blisters all over. Place chiles briefly in plastic bag to steam as they cool.
When chiles are cool enough to handle, peel them, wearing rubber gloves if your skin is sensitive. Slit each chile from end to end and remove any loose seeds. Don’t remove seed pod or it will weaken chile walls. (Chiles can be prepared to this point a day ahead, covered, and refrigerated. Bring them back to room temperature before proceeding.).
Stuff chiles with filling, bringing slit edges of each chile tightly back together. Spritz chiles with oil. Return chiles to grill, slit side down. Grill uncovered 5 to 7 minutes, turning once, until filling feels quite soft. Serve chiles hot with drizzle of sauce and garnish of chopped tomato.