Step 1Prepare the ajo blanco: Cut the bread into 3 pieces, transfer to a bowl and add enough water to soak the bread. Toss to coat, then let sit while you prepare the garlic.
Step 2Add the garlic cloves to a small saucepan and cover with water. Bring to a boil over high, then drain. Repeat two more times, adding fresh water each time then draining after it comes to a boil.
Step 3Add the garlic cloves to a blender, along with the olive oil, cashews, vinegar and a pinch of salt. Give the bread a gentle squeeze to remove excess water, then add the bread to the blender. Purée the mixture until very smooth, scraping the sides as needed, then season to taste with salt. It may not emulsify immediately, but be patient with your blender. If the mixture seems too thick, you can add a splash of water, if needed. (If you’ve got an immersion blender, you may want to employ it here for extra-silky results.) Set aside ajo blanco.
Step 4Prepare the broccoli rabe: Bring a large pot of generously salted water to a boil. Add the broccoli rabe and cook just until bright green, then transfer to a colander to drain. Pat the broccoli rabe dry using a clean kitchen towel.
Step 5Heat 2 tablespoons olive oil in a large cast-iron skillet over high. Working in three batches, add the broccoli rabe in a single layer, making sure not to crowd the pan. Cook, turning once, until charred on two sides, about 2 minutes per side. Remove from the skillet and transfer to a platter. Repeat two more times, adding 1 tablespoon olive oil to the skillet before each new batch, plus more if needed.
Step 6Once all of the broccoli rabe has been charred, add the lemon halves to the skillet, cut sides down, and cook over high heat until charred, about 2 minutes.
Step 7Spoon the ajo blanco onto plates and top with the broccoli rabe.
Step 8In a small skillet, toast the grated garlic in 2 tablespoons olive oil over medium heat, stirring frequently, until the morsels dry out, separate and start to look golden, about 2 minutes. Stir in the red-pepper flakes, then pour the seasoned oil over the rabe. Serve with the charred lemon halves, for squeezing on top.
