Preheat oven to 425 degrees. On two rimmed baking sheets, arrange green beans, onions, and mushrooms. Season with salt and pepper and toss each with 1/4 cup olive oil.
Roast for 35 minutes until tender and browned.
Meanwhile, in a medium skillet over medium heat, heat remaining 1/4 cup oil. Add bread crumbs and oregano and cook, stirring constantly for 3 minutes, until golden brown. Turn off heat and stir in Parmesan and zest of lemon.
Top with Parmesan bread crumbs and then squeeze lemon juice over roasted vegetables.