Sauté the aromatics. Heat 2 tablespoons of extra-virgin olive oil in a large skillet over medium heat. Add the diced yellow onion and cook for 3 to 4 minutes until soft and translucent. Add the minced garlic, chopped rosemary, and crushed red pepper flakes; stir constantly for 1 minute until fragrant without letting the garlic brown.
Braise the greens. Add the sliced broccoli rabe stems and chopped leaves to the skillet along with the vegetable broth. Cover the pan loosely and simmer over medium heat for 5 to 6 minutes, stirring halfway through, until greens turn deep green, wilt completely, and stem pieces are easily pierced with the tip of a paring knife.
Simmer and lightly mash. Stir in the drained cannellini beans, salt, and black pepper. Use the back of a wooden spoon or potato masher to crush about half of the beans against the bottom of the pan to create a creamy base. Cook uncovered for 2 to 3 minutes, stirring occasionally, until the liquid reduces to a glossy glaze coating the beans and greens.
Finish with citrus. Remove the skillet from heat and stir in the lemon zest and fresh lemon juice. Taste and adjust with an extra pinch of salt or pepper flakes if desired.
Prepare the garlic toast. Toast the sourdough slices in a toaster or under a preheated broiler for 2 to 3 minutes per side until deep golden brown and sturdy. Immediately rub the cut side of the halved raw garlic clove across the rough top surface of each warm toast, then brush lightly with 1 tablespoon of extra-virgin olive oil.
Spoon and serve. Divide the warm bean and broccoli rabe mixture evenly among the garlic toasts. Drizzle each slice with extra-virgin olive oil and serve immediately.