Warm vegetable broth in a medium saucepan over medium-low heat. Keep warm.
While the vegetable broth is warming, heat olive oil over medium heat in a large sauté pan. Add onions and garlic and cook until fragrant. Add mushrooms and cook for 1-2 minutes. Add arborio rice and cook for 1-2 minutes. You should notice a nutty aroma. Add sherry to sauté pan and cook until it is mostly absorbed.
Add warm vegetable broth to the sauté pan one cup at a time. Stir and allow the broth to mostly absorb into the rice between each cup.
After the last cup of broth is added and absorbed, add butter and stir vigorously. Once the butter has melted, repeat with the cheese. The mixture should be creamy. Add salt and pepper to taste. Cover and keep warm.
Make the broccoli rabe purée by combining all purée ingredients in a blender. Blend until smooth.
Add the purée to the risotto and stir to combine. Garnish with shaved parmesan cheese and fresh oregano and serve warm.