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1
Bring a large pot of salted water to a boil. Add the pasta and cook until quite al dente, about 2 minutes short of the recommended time on the package.
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2
Meanwhile, heat 3 tablespoons of oil in a large skillet over medium-high. When the oil is hot, add the sprig of thyme and let it sizzle. When it's changed color and shriveled, 1 to 2 minutes, add the hazelnuts. Cook, stirring often, until golden brown, 2 to 3 minutes, then use a slotted spoon to transfer nuts and thyme to a paper towel-lined plate. Sprinkle with salt.
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3
Return the skillet to medium-high heat. Add the red pepper flakes and black pepper and let cook until fragrant, about 1 minute. Turn the heat down to low.
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4
By now, the pasta should be finished. Scoop out 1 cup of pasta water to keep for the sauce, then drain the pasta and transfer to the skillet. Add the garlic and stir to combine. Mix in a couple tablespoons of pasta water, then sprinkle over a few tablespoons of pecorino and mix vigorously with a wooden spoon to melt in the cheese. Continue adding pasta water and cheese and mixing with verve until smooth and combined.
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5
Add the butter and mix until you have a glossy sauce, then mix in the zest and juice from half a lemon. Mix in the reserved hazelnuts and thyme. Taste for salt and lemon.
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6
Top with additional fresh thyme, black pepper, and grated pecorino.
