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1
Combine lemongrass, garlic, chiles, ginger, and lime zest in a food processor. Process until a fine paste forms.
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2
Place beef in a bowl and season with salt and pepper. Heat oil in a large Dutch oven over medium-high heat. Cook beef, turning occasionally, until browned, 10 to 15 minutes. Transfer beef to a plate.
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3
Cook lemongrass paste in the same Dutch oven stirring often, until beginning to soften, 5 to 8 minutes. Add soy sauce, brown sugar, fish sauce, browned beef and any juices, and 10 cups water. Bring to a boil and reduce heat. Simmer stew, partially covered and skimming occasionally, until beef is tender and liquid is slightly thickened, 2 1/2 to 3 hours.
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4
Meanwhile, preheat the oven to 350 degrees F (175 degrees C). Place coconut flakes on a rimmed baking sheet.
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5
Toast coconut in the preheated oven, tossing occasionally, until golden around the edges, about 4 minutes. Set aside.
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6
Add carrots and shallots to the stew. Continue cooking, partially covered, until vegetables are soft and beef is falling apart, 35 to 45 minutes more. Stir in green onions.
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7
Bring a large pot of lightly salted water to a boil. Cook rice noodles in boiling water, stirring occasionally, until noodles are tender yet firm to the bite, 5 to 7 minutes, or according to package directions.
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8
Divide cooked noodles into individual bowls and ladle stew on top. Top with toasted coconut and serve with lime wedges.