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.
Heat a grill or grill pan over medium-high heat until hot; brush with the remaining 1/2 tablespoon oil. Season the steak with salt and pepper. Grill the steak, turning once, 3 to 4 minutes per side for medium rare. Transfer to a cutting board and let rest for 5 minutes, then cut into 1/2-inch pieces.
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 steak, onion, celery, parsley and tomatoes 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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