Bring a large pot of salted water to a boil. Meanwhile, heat butter in a 2-quart heavy saucepan over medium-low heat until melted, then add flour and cook over low heat, whisking, 3 minutes (do not brown). Add hot milk little by little, whisking to incorporate between additions, then whisk in 3/4 teaspoon salt and add 1 garlic clove. Bring to a boil, whisking, then reduce to a simmer and cook, whisking constantly, until béchamel is thickened, about 4 minutes. Remove and discard garlic. Remove béchamel from heat, and season with pepper to taste. Cover surface of sauce with parchment or wax paper.
 

Cook broccoli rabe in the boiling salted water, uncovered, until just tender, about 5 minutes. Using tongs or a slotted spoon, transfer to a large bowl (reserve boiling water). Drain well in a colander and let cool slightly, then pat dry and roughly chop. In a large skillet, heat oil and remaining garlic clove over medium heat; add broccoli rabe and cook, stirring, for 10 minutes. Remove from heat; remove and discard garlic.
 

Cook pasta in the boiling water until al dente, then drain. In the prepared baking dish or dishes, create 2 layers each of pasta, béchamel, broccoli rabe and cheese, and a third layer of béchamel and cheese. Bake until bubbly, about 12 minutes, then brown under the broiler until golden, about 2 minutes. Serve immediately.