Slideshow: More Great Sandwiches
Preheat the oven to 350°. Bring a saucepan of water to a boil. Add the broccoli rabe and cook until softened, about 5 minutes. Drain and rinse under cold water. Pat dry and coarsely chop the broccoli rabe.
Heat the olive oil in a large ovenproof nonstick skillet. Add the garlic and cook over moderately high heat, stirring, until lightly browned, about 1 minute. Add the broccoli rabe and cook until lightly browned in spots, about 3 minutes. Add the spinach, season with salt and pepper and cook, stirring, until wilted, about 1 minute; discard the garlic.
In a medium bowl, whisk the eggs with the milk and season with salt and pepper. Add the mozzarella. Pour the eggs over the greens and shake the skillet gently to settle the mixture. Transfer the skillet to the oven and bake the frittata for about 20 minutes, or until set and puffed. Cut the frittata into quarters and sandwich between the slices of toasted brioche. Cut each sandwich in half and serve.