Heat a large nonstick skillet over medium heat and line a plate with paper towels. Add the prosciutto slices (you may need to do this in batches), and cook until browned and crisp, about 2 minutes on each side. Transfer to the paper towels to drain. The prosciutto will become crisper as it cools. Add enough oil to make 2 tablespoons fat in the pan and heat over medium-high heat for 1 minute. Add the broccoli rabe. Cook, stirring occasionally, until soft, tender, and bright green yet caramelized around a few of the edges, 5 to 7 minutes. (Add water to the pan if it seems dry.) Add the lemon juice, pepper flakes and salt and toss to coat. Transfer the broccoli rabe to a plate. Wipe out the skillet but don't wash it. Brush the remaining oil on one side of each slice of bread. Place 4 slices of bread, oil side down, on your work surface. Distribute the broccoli rabe and follow with the prosciutto. Pile the cheese on top, compressing it with your hand if necessary, and top with the remaining bread slices, oil side up. Reheat the skillet over medium heat for 2 minutes. Put the sandwiches into the pan, cover and cook for 3 to 4 minutes, until the undersides are golden brown. Turn the sandwiches, pressing each one firmly with a spatula to flatten slightly. Cover and cook for 2 to 3 minutes or until the undersides are well browned. Remove the cover, turn the sandwiches once more and press firmly with the spatula once again. Cook for 1 minute or until the cheese has melted completely. Remove from the pan and let cool 2 to 3 minutes. Cut in half and serve.