In a blender, combine tightly packed fresh parsley, dill, tarragon, basil, chives, lemon juice, and garlic. Add mayonnaise and sour cream, then blend until smooth and creamy, about 30 seconds. Taste the dressing and season with salt and pepper to your liking.

Remove the outer leaves from the Savoy cabbage to expose the fresh interior. Halve the cabbage, core it, and slice it thinly, creating delicate ribbons. In a large mixing bowl, combine the shredded cabbage with thinly sliced sweet onion.

Pour the creamy dressing over the cabbage and onion mixture. Toss everything gently to combine until all the ingredients are coated. Cover and chill in the refrigerator for at least 30 minutes.