Step 1In a saute pan over high heat, toast the whole cumin, tossing constantly until it begins to smoke slightly. Allow to cool.
Step 2Grind the cumin in a spice grinder until completely ground.
Step 3Place the lime juice and sour cream in a blender. Add the cumin, salt, black pepper, cayenne and sugar. Blend for 30 seconds.
Step 4Add the green onions, cilantro and chervil and blend for another 30 seconds. Gradually blend in the canola oil until emulsified. Taste and adjust seasoning as necessary. Refrigerate.