4 large shallots (1/2 pound), thinly sliced lengthwise
6 large garlic cloves, thinly sliced lengthwise
3/4 tsp. kosher salt, divided
2 Tbsp. butter
24 large shrimp (about 1 1/4 pounds), shelled and deveined
2 Tbsp. fresh lime juice
2 scallions, thinly sliced
1 tsp. yellow mustard seeds, toasted, for garnish (optional)
Lime wedges, for serving
Active time: 25 minutes
Total time: 45 minutes
In a small bowl, combine cumin, cardamom, ground mustard, turmeric, and red pepper flakes. Add curry leaves and fenugreek, if using.
In a large skillet, heat oil over medium heat. Add shallots and garlic and cook, stirring occasionally, until they start to soften, about 2 minutes. Add 2 Tbsp. water, cover, and reduce heat to low, stirring a few times, until golden brown and very soft, about 10 minutes more. Add 1/4 teaspoon salt and 1 tsp. spice mixture and cook over medium heat, stirring, until fragrant, about 2 minutes. Scrape mixture into a medium bowl. Return skillet to stove.
Add butter to skillet and cook over medium heat until starting to brown, 1 to 2 minutes. Add shrimp in an even layer and season with 1/4 tsp. salt. Cook over medium heat until shrimp start to curl, about 1 minute per side. Sprinkle with 2 tsp. spice mixture and continue to cook, stirring, until fragrant and shrimp are almost cooked through, about 2 minutes more. Stir in shallot-garlic mixture and lime juice and simmer until heated through, about 1 minute more. Season with remaining 1/4 teaspoon salt.
Spoon shallot-garlic mixture onto 6 small plates. Add 4 shrimp to each plate and scatter on top scallions and mustard seeds, if using. Serve with lime wedges.