Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
Combine the beans and mushrooms in a large bowl.
Whisk together the lemon juice, olive oil, salt, and dried thyme, and pour over the beans and mushrooms. Toss with your hands until the vegetables are thoroughly coated.
Spread on the prepared baking sheet and bake for 30 minutes. Remove from the oven and sprinkle the French fried onion ring crumbles and Parmesan cheese evenly over the roasted vegetables. Roast for another 10 minutes until the cheese is melted.