Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add 1 bunch chopped broccoli rabe; season with salt, pepper and red pepper flakes. Cook until the broccoli rabe is wilted, about 5 minutes. Toast 1 sliced baguette in a 350 degrees F oven, 6 to 8 minutes. Top each toast with a thin slice of fresh mozzarella and the broccoli rabe. Drizzle with aged balsamic vinegar.