1. 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.
2. In another large bowl, toss the shrimp with the oil, garlic, chile
powder and salt and let stand for 10 minutes.
3. 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.
MAKE AHEAD The miso butter can be refrigerated in an airtight
container for up to 3 days or frozen for up to 1 month.
SERVE WITH Grilled scallions and steamed white or brown rice.