The day before, get together with a friend. One of you makes the dressing: Purée black garlic, mayonnaise, sour cream, vinegar, and 31/2 tbsp. salt in a blender. Turn mixture into a bowl and stir in tarragon, parsley, and chives, adding more salt to taste if you like. Chill.
The other person makes the tamarind giardiniera: Put shallots and chiles in a large heat-proof bowl. In a 4- to 6-qt. saucepan, bring sugar, vinegar, chile flakes, salt, tamarind concentrate, and 2 cups water to a boil, stirring occasionally to dissolve sugar. Pour boiling liquid over shallots and chiles. Let cool.
Working together, prep the vegetables for the salad. Store radishes in 1 resealable bag and cabbage in 6 of them (1 per platter). Store herb leaves in resealable bags lined with paper towels.
In a large bowl, mix giardiniera with sliced radishes, sesame oil, and grapeseed oil. Chill.
About 2 hours before serving, get help cutting nectarines: Slice into 1/2- in.-thick wedges and add to giardiniera. Chill 30 minutes, then drain and divide into 6 equal batches.
One of you dresses and plates the cabbage and the other adds nectarines and herbs: Put 1 batch sliced cabbage in a large bowl and dollop with 1 cup black garlic dressing. Toss well to mix (cabbage should be thickly coated), then turn out onto a platter. Repeat for remaining 5 batches.
Arrange nectarine mixture over cabbage and sprinkle with herbs.
*Find black garlic at well- stocked grocery stores, and easy-to-peel single-clove RioRand Yu Hong Yuan black garlic on amazon. com ($14.99 for a 170-g jar). is a smooth, dense, Vietnamese-style tamarind concentrate found at Asian markets.