Did you Google "mustard frills"? I did and here is the page of results:
mustard frills - Google Search
It's the name of the crop.
Wadmalaw, Johns and other barrier islands lie between Charleston and high-end resort islands like Kiawah and Seabrook; many farms here have been sold because the rich dirt is more profitable for raising second homes than vegetables.
But there are holdouts like Mr. Thackeray who are hoping to ride the Southern revival to sustainability, if not wealth, by raising chef-friendly crops like mustard frills, Ragged Jack kale and Bright Lights chard, organic peanuts and watermelon radishes. Celeste Albers, a local legend for her knowledge of traditional agriculture, has dreams of producing buttermilk for sale from her herd of Jersey cows.
(Southern Farmers Vanquish the Clichés. The NY Times)
What does "frills" mean in this sentence?
The sentence lists "chef-friendly" crops.
Mustard is a plant. The frills must be the part of the plant that is needed to make mustard.