Preheat oven to 400 degrees F. Line a large baking sheet with parchment paper and spray with nonstick cooking spray. Set aside.
Place sliced onions in a large bowl, drizzle with 1 tablespoon of olive oil then add in flour, panko bread crumbs, garlic powder and salt. Use your hands to toss to combine, then place onions onto prepared baking sheet and bake for 15-25 minutes, using a tong to toss the onions every 8-10 minutes or so, until nice and crispy.
Once done cooking, set onions aside. Reduce the heat in the oven to 350 degrees F.
While the onions are cooking, bring a large pot of salted water to a boil. Add green beans in two batches, cooking for about 2-3 minutes per batch. I like to remove the green beans from the water with metal tongs and transfer to a large bowl, then repeat with additional green beans. Set aside.
Next cook your mushrooms: Place a large skillet over medium-high heat and add in 2 tablespoons butter. Once butter melts, add the mushrooms, salt, and pepper. Cook mushrooms, stirring occasionally, until all liquid is absorbed from mushrooms and they are golden brown; 8 to 10 minutes. Then stir in the thyme, red pepper flakes, and garlic and cook, stirring until fragrant, 1 minute.
In the same skillet you cooked the mushrooms, add in 2 tablespoons of butter and place over medium heat. Once butter melts, whisk in flour and cook for 15-30 seconds until a paste forms, then VERY slowly add 2 cups of the broth, while you continue to whisk away any lumps. You’ll need to whisk pretty vigorously.
Once smooth, increase heat, and bring mixture to a boil, then reduce heat and simmer for 5 minutes stirring every so often until sauce thickens up a bit, similar to a gravy. If it gets way too thick, add in ¼-½ cup more broth.
Once thick, turn off the heat and stir in minced garlic and parmesan cheese. Season with salt and pepper to taste.
Spray a 2 quart baking disk or 9x13 inch baking pan with nonstick cooking spray, or grease with olive oil or butter. Add in green beans, mushrooms and 1/4 of the crispy onions; gently toss together.
Pour the sauce all over, then cover with foil and bake for 25-30 minutes. Remove foil, add the remaining crispy onions on top and bake for 5-10 more minutes. Serves 8-10