Sauté the garlic and chile flakes in the olive oil over medium-high heat in a large skillet or dutch oven until fragrant, about 2 minutes. Add the broccoli rabe and salt; sauté until just starting to color, about 2 to 3 minutes more. Add 1/2 cup water to the pot, cover, and gently simmer for 1 hour. Give the broccoli rabe a stir a few times throughout the process. If the pan is dry add just a bit of water to keep the broccoli rabe from scorching.
After an hour the broccoli rabe will be very tender and slumped. Finish with just a squeeze of fresh lemon.
Butter one side of each slice of bread.
Preheat a large skillet over medium-high heat. Add four slices of bread buttered side down, and top the bread with 1/4 of the broccoli rabe. On top of that add roughly 1/2 cup grated cheese. Top with another slice of bread, this time butter side up.
Grill until golden and the cheese has melted, about 3 minutes per side. Give the sandwich a gentle press with a spatula to compress the sandwich so the broccoli rabe and cheese really meld together. Serve immediately.