Combine the onions, flour, panko, olive oil, and salt in a large mixing bowl. Spray a baking sheet with nonstick cooking spray and evenly spread out the coated onions. Bake onions on the medium rack until golden brown. This takes about 30 minutes make sure to toss the onions 2 to 3 times during cooking to ensure they don’t burn. Once done, remove from the oven and set aside until ready to use.
Beans and Creamy Mushroom Sauce
Blanch the trimmed green beans and set aside for later
Caramelize the onion in 1 tbsp of butter with a pinch of salt for about 5-7 minutes. Once caramelized remove them and set them aside.
In the same pan, the onions were cooked in, place the remaining butter and sliced mushrooms with a pinch of salt, DO NOT CROWD PAN. Cook mushrooms 3-5 minutes stirring occasionally
Add the garlic and thyme give it a quick stir. Stir in the flour and allow to cook for 1 minute. Slowly whisk in the stock, and the cream. Taste and add salt and pepper to taste. Once the sauce has thickened stir in your green beans. Top with crispy onions bake for 10 minutes and serve.
You can make this Cast Iron Green Bean Casserole up to 5 days ahead. Blanch beans and place in a bag with a paper towel to help draw out any additional moister. Make sauce and allow to cool then place the mushroom sauce in a mason jar or any glass jar with a tight lid. The onion topping, I would make the same day to avoid any sogginess from occurring, or you can use over the counter to make your life easier. The day you are ready to serve mix beans and mushroom sauce, top with onion crispies and place in the oven for 15-20 minutes to warm through.