For the Crispy Onions:
2 In a large bowl, combine the sliced onion, ½ cup cassava flour, 1 teaspoon onion powder and 1 teaspoon garlic powder and toss to coat.
3 Add the olive oil to a frying pan and heat over medium heat until the oil reaches around 360F. Working in batches, dust off the excess flour from the onions and add them to the hot oil. Cook, flipping occasionally, until golden brown and crispy all over, around 8 minutes. Transfer the fried onions to a paper towel lined plate to drain excess oil. Season with a pinch of salt and set aside.
For the Green Bean Casserole:
6 Preheat oven to 350F.
7 Bring a large pot of water to a boil. Add the green beans and cook until slightly tender and bright green, around 3 minutes. Transfer the green beans to a bowl of ice water for 5 minutes to stop the cooking and set the colour. Drain the water and set the green beans aside.
8 Preheat a large oven-safe pan over medium heat. Add the olive oil and heat until shimmering. Add the chorizo and cook, stirring occasionally, until browned and slightly crispy, around 8 minutes. Reserving the rendered fat in the pan, transfer the chorizo to a paper towel-lined plate and set aside.
9 To the pan, add the mushrooms and cook, undisturbed, until golden on the bottom, around 5 minutes. Continue cooking the mushrooms, stirring occasionally, until slightly softened, 2 more minutes. Add the butter and heat until melted. Add the shallots and leek and season with a pinch of salt. Cook, stirring, until the leeks are soft and golden, around 8 minutes. Add the garlic and cook, stirring, 60 seconds. Add 1 tablespoon cassava flour and cook, stirring, 60 seconds. Pour in the coconut milk and cook, stirring, until reduced in volume by half, around 3 minutes. Add the mustard, thyme, ½ teaspoon onion powder, and ½ teaspoon garlic powder and cook, stirring, 60 seconds.
10 Pour in the beef bone broth and cook, lifting any brown bits off the bottom of the pan, until the broth has reduced by 1/2 in volume, around 6 minutes. Taste for seasoning and adjust with salt and pepper as desired.
11 Add the blanched green beans to the pan and toss to coat in the sauce. Transfer the pan to the oven and cook, uncovered, until the green beans are tender, around 25 minutes. Top the casserole with the cooked chorizo and crispy onions and cook an additional 5 minutes. Remove from the pan from the oven and let sit 10 minutes before serving.
