Preheat oven to 400°F. Coat a 2 1/2-quart baking dish with avocado oil

Heat 1 Tbsp  oil in a large saucepan over medium heat. Add diced onion and cook, stirring often, until softened and slightly translucent, about 4 minutes.

Stir in mushrooms, onion powder, 1 tsp salt, thyme and pepper. Cook, stirring often, until the mushroom juices are almost evaporated, 3 to 5 minutes.

Sprinkle 1/3 cup flour over the vegetables; stir to coat. Add milk and sherry and bring to a simmer, stirring often.

Stir in green beans and return to a simmer. Cook, stirring, until heated through, about 1 minute. Stir in sour cream and buttermilk powder. Transfer to the prepared baking dish.

Whisk the remaining 1/3 cup flour, paprika, garlic powder and the remaining 1/4 tsp  salt in a shallow dish.

Add sliced onion; toss to coat. Heat the remaining 2 Tbsp oil in a large nonstick skillet over medium-high heat.

Add the onion along with any remaining flour mixture and cook, turning once or twice, until golden and crispy, 4 to 5 minutes. Spread the onion topping over the casserole.

Bake the casserole until bubbling, about 15 minutes.

Let cool for 5 minutes before serving.