In a small saucepan, boil the vinegar over medium-high heat until reduced by half, about 4 minutes. Set aside.

Bring a large pot of well-salted water to a boil over high heat. Have a large bowl of ice water ready. Blanch the broccoli raab in the boiling water for 3 minutes (the water needn't return to a boil). Drain it and transfer to the ice water to cool. Drain well and gently squeeze the broccoli raab to remove excess water.

Heat the garlic and oil in a large skillet over medium heat until the garlic begins to turn golden, 2 minutes; remove and discard the garlic. Add the anchovies, mashing them with the back of a wooden spoon until fragrant, 30 seconds. Add the pepper flakes and cook, stirring constantly, for 5 to 10 seconds. Add the broccoli raab and cook, stirring often, until tender and heated through, 3 to 4 minutes. Drizzle with the vinegar and season to taste with salt and pepper. Transfer to a platter and serve.

You can reduce the vinegar and blanch the broccoli raab up to 6 hours ahead. Leave the vinegar at room temperature and refrigerate the broccoli raab, returning it to room temperature before finishing the dish.