Preheat oven to 425 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.
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 ½ 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 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.
To make gluten free: use gluten free breadcrumbs and gluten free all purpose flour.