Heat 1/3 cup of the oil in a medium skillet over medium heat. Add the garlic and red pepper flakes and cook, stirring, until the garlic is golden, about 3 minutes. Transfer to a large bowl and cool to room temperature. Stir in the lemon juice and season with 1/2 teaspoon salt and a few grinds of pepper.
Meanwhile, bring a large pot of salted water to a boil.
Cook the asparagus in the boiling water until crisp-tender, about 3 minutes. Transfer the asparagus to a colander with a slotted spoon and run under cold water to stop the cooking process. Add to the bowl with the garlic oil.
Add the pasta to the boiling water and cook according to the package directions. Drain and rinse under cold water to cool; add to the bowl.
Add the roast beef and scallions to the bowl and toss to combine. Season with salt and pepper. The pasta salad is best if served right away.
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