Prepare a large bowl of ice water. Bring a pot of salted water to a boil and cook the broccoli rabe until slightly tender, 3 to 4 minutes. Transfer to the ice bath for 2 minutes to stop the cooking, then remove and dry off.
Add the olive oil to a large saute pan and heat over medium heat until shimmering. Add the panko and toast until golden, about 1 minute. Remove from the pan.
Add the anchovy paste and let it melt in. Add the broccoli rabe, plus more olive oil if dry, along with the red pepper flakes and some salt and pepper. Cook until tender, about 5 minutes. Remove from the heat and add the lemon zest and juice.
Add the broccoli rabe to a serving bowl and top with the crispy panko.