How to Make It
In a food processor, combine the butter with the miso, lemon zest and lemon juice and puree until smooth. Add the 1 tablespoon of scallion and pulse just until incorporated. Scrape the miso butter into a large bowl and set aside.
In another large bowl, toss the shrimp with the oil, garlic, chile powder and salt and let stand for 10 minutes.
Light a grill or preheat a grill pan. Grill the shrimp over high heat, turning once, until just cooked through, about 4 minutes. Immediately add the shrimp to the miso butter and toss until well coated. Garnish the shrimp with scallions and the pickled mustard seeds and brine and serve.
The miso butter can be refrigerated in an airtight container for up to 3 days or frozen for up to 1 month.
Grilled scallions and steamed white or brown rice.