3 Tbsp. Fish Sauce
1 cup unseasoned Rice Vinegar
¼ cup sliced Scallions
1 ½" piece Ginger, peeled, finely grated
Kosher Salt
2 tbsp. Aleppo-style Pepper, or 1 Tbsp. Crushed Red Pepper Flakes
¾ cup Unsalted, Roasted Peanuts, plus ¼ cup crushed
Kosher Salt
1 lb. Brussels Sprouts, preferably small ones, trimmed, divided
1 cup Vegetable Oil
6" Corn Tortillas
¼ cup finely chopped White Onion
Sliced Avocado, Sliced Serrano Chiles, Cilantro Leaves with tender stems, flaky Sea Salt, and Lime Wedges (for serving)
Heat a small saucepan over medium. When hot, pour in 1 Tbsp. fish sauce. It should bubble up vigorously and then get thicker and slightly darker, about 30 seconds. Repeat with remaining fish sauce, incorporating 1 Tbsp. at a time. Carefully add vinegar (it may spatter), then transfer mixture to a small bowl; let cool. Stir in scallions and ginger. Taste and season with salt.
Purée red pepper and ¾ cup whole peanuts in a food processor until a smooth butter forms, about 2 minutes. Season with kosher salt and scrape into a small bowl.
Very thinly slice one-fourth of brussels sprouts; transfer to a medium bowl. Cut remaining brussels sprouts in half. Heat oil in a medium skillet over medium-high. Shallow-fry halved brussels sprouts in batches, being careful not to overcrowd the pan, until browned and crisp, about 5 minutes per batch. Transfer to paper towels to drain, then add to sliced brussels sprouts. Drizzle with ¼ cup Salty-Sour Vinaigrette and toss to coat; season with kosher salt.
To serve, heat a medium skillet over medium-high. Working one at a time, cook tortillas just until warmed through; transfer to plates. Spread tortillas with some spicy peanut butter; spoon brussels sprouts over. Top with onion, avocado, chiles, cilantro, and ¼ cup crushed peanuts; sprinkle with sea salt. Serve with lime wedges.