For the Green Bean Casserole:

1. Preheat the oven to 375°F.
2. If you are using frozen green beans: remove from the freezer 1 hour before cooking to bring to room temperature or follow the package instructions for defrosting. Drain, pat dry, and transfer to a 9×13” baking dish. Set aside.
3. If you are using fresh green beans: bring a pot of water to boil. Grab a large bowl and fill it with ice water. Add the green beans to the boiling water and cook for 3 minutes until bright green and tender. Transfer to the ice bath to stop the cooking process. Once cool, about 1 minute, drain and transfer to a 9×13” baking dish. Set aside.
4. Now work on your mushroom cream sauce. Heat a large skillet on medium high heat and add in your oil. Add the mushrooms and cook undisturbed for about 2 minutes or until browned underneath. Toss and cook for another 2-3 minutes or until browned all over. You can do this in batches to create even browning. 
5. Add 1 tbsp of butter to the mushrooms and a pinch of salt. Toss, cooking for another minute. Remove from the skillet and set aside.
6. Add 1 tbsp butter to the skillet and add in your minced shallot. Cook for 2-3 minutes or until soft, then add in your garlic and cook for 30-60 seconds more.
7. Add in 2 tbsps of butter and stir until melted. Reduce the heat to low, whisk in your flour, and cook for about 2-3 minutes, whisking continuously, until golden brown and fragrant. 
8. Slowly stream in the chicken or vegetable broth while whisking constantly until smooth. Whisk in the heavy cream and cook on medium heat until thickened and bubbling, about 4-5 minutes.
9. Whisk in the dried thyme, mushroom powder, and dijon mustard. Shut off heat and whisk in the parmesan. Add back in the mushrooms, stir, and taste to adjust for seasoning.
10. Pour mushroom cream mixture over the green beans and stir to combine. It may not look like enough at first, but it will get saucier as it bakes.
11. Transfer to a 375°F oven and bake uncovered for 15 minutes. Top with the crispy fried onions and bake for an additional 6-8 minutes or until the onions are golden brown and crispy. Serve immediately and enjoy!